chore: removing unused css (#30214)

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## **Description**

This PR removes unused CSS classes and files from the codebase to
improve maintainability and reduce CSS size by `~30kb`. The changes
include:

1. Removing unused CSS files:
   - network.scss
   - send.scss
2. Removing unused CSS classes from newui-sections.scss:
   - .lap-visible/.phone-visible classes
   - .fiat-amount
   - .token-balance__amount
   - .unlock-screen-container and related classes

[![Open in GitHub
Codespaces](https://github.com/codespaces/badge.svg)](https://codespaces.new/MetaMask/metamask-extension/pull/30214?quickstart=1)

## **Related issues**

Fixes: MetaMask/MetaMask-planning#4172

## **Manual testing steps**

1. Pull the branch and build the extension
2. Test all major UI flows to ensure no visual regressions:
   - Onboarding flow
   - Send flow
   - Network switching
   - Unlock flow
3. Verify removed classes are not used in any components
4. Check browser dev tools to confirm CSS bundle size reduction

## **Screenshots/Recordings**

### **Before**
![Screenshot 2025-02-07 at 12 10
41 PM](https://github.com/user-attachments/assets/24e48b05-dbaa-485e-8523-6189094d547e)

### **After**

![Screenshot 2025-02-07 at 12 08
17 PM](https://github.com/user-attachments/assets/3b5c71d0-4ee3-41fb-890a-5c16400a4cdd)


Not visual regressions to the following flows"

**Onboarding Flow**


https://github.com/user-attachments/assets/c6e5d87d-a2d3-4a3c-be6a-096db388f854

**Send Flow**


https://github.com/user-attachments/assets/3b5f37a1-262d-4a5c-b891-5b9c158958fb

**Changing Network Flow**


https://github.com/user-attachments/assets/98bc52d5-edc1-4f52-a91c-44621df25ec6

**Unlock flow**


https://github.com/user-attachments/assets/acde49ca-7273-4c0b-929c-94206e2c8267

## **Pre-merge author checklist**

- [x] I've followed [MetaMask Contributor
Docs](https://github.com/MetaMask/contributor-docs) and [MetaMask
Extension Coding
Standards](https://github.com/MetaMask/metamask-extension/blob/main/.github/guidelines/CODING_GUIDELINES.md).
- [x] I've completed the PR template to the best of my ability
- [x] I've included tests if applicable
- [x] I've documented my code using [JSDoc](https://jsdoc.app/) format
if applicable
- [x] I've applied the right labels on the PR (see [labeling
guidelines](https://github.com/MetaMask/metamask-extension/blob/main/.github/guidelines/LABELING_GUIDELINES.md)).
Not required for external contributors.

## **Pre-merge reviewer checklist**

- [ ] I've manually tested the PR (e.g. pull and build branch, run the
app, test code being changed).
- [ ] I confirm that this PR addresses all acceptance criteria described
in the ticket it closes and includes the necessary testing evidence such
as recordings and or screenshots.

test/e2e/tests/transaction/send-eth.spec.js

@@ -48,10 +48,6 @@ describe('Send ETH', function () {
           await inputAmount.press(driver.Key.BACK_SPACE);
           await inputAmount.press(driver.Key.BACK_SPACE);
 
-          await driver.assertElementNotPresent('.send-v2__error-amount', {
-            waitAtLeastGuard: 100, // A waitAtLeastGuard of 100ms is the best choice here
-          });
-
           const amountMax = await driver.findClickableElement(
             '[data-testid="max-clear-button"]',
           );

ui/css/itcss/components/index.scss

@@ -1,4 +1 @@
-@import './network';
 @import './newui-sections';
-@import './send';
-

ui/css/itcss/components/newui-sections.scss

@@ -50,14 +50,6 @@
 // main-container media queries
 
 @include design-system.screen-sm-min {
-  .lap-visible {
-    display: flex;
-  }
-
-  .phone-visible {
-    display: none;
-  }
-
   .main-container {
     width: 85vw;
     min-height: 90vh;
@@ -82,14 +74,6 @@
 }
 
 @include design-system.screen-sm-max {
-  .lap-visible {
-    display: none;
-  }
-
-  .phone-visible {
-    display: flex;
-  }
-
   .main-container {
     width: 100%;
     overflow-y: auto;
@@ -101,14 +85,6 @@
   }
 }
 
-.fiat-amount {
-  text-transform: uppercase;
-}
-
-.token-balance__amount {
-  padding: 0 6px;
-}
-
 // first time
 .first-view-main {
   display: flex;
@@ -132,24 +108,6 @@
   }
 }
 
-.unlock-screen-container {
-  z-index: design-system.$main-container-z-index;
-  display: flex;
-  justify-content: center;
-  align-items: center;
-  flex: 1 0 auto;
-  background: var(--color-background-alternative);
-  width: 100%;
-}
-
-.unlock-screen {
-  display: flex;
-  flex-direction: column;
-  justify-content: center;
-  align-items: center;
-  flex: 1 0 auto;
-}
-
 .first-view-main-wrapper {
   display: flex;
   width: 100%;