Revert "Enable HOMEBREW_AUTOREMOVE by autoremove_default"

[carlocab]

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autoremove is destructive, and it can be difficult for users to
recover from it occuring when they weren't expecting it.

Fixes #17363
Fixes Homebrew/discussions#5395

This reverts commit 3d11416.
This reverts commit efb14a0.

[MikeMcQuaid]

This should not have been merged so quickly (<4 hours) without further discussion based on two reports (one of which was immediately banned) on a weekend.

We announced this as a breaking change in our release notes. Let's figure out how to improve the feature rather than yo-yoing on functionality like this.

[MikeMcQuaid]

https://github.com/orgs/Homebrew/discussions/5395

It was either not manually installed or there's a bug here that needs fixed with brew autoremove, too.

#17363

In both of these cases: the solution is to run a brew install <formula>. I'm not seeing what's hard about doing so?

Note this is pretty similar behaviour to brew cleanup which will also remove cached downloads the user may soon need similarly aggressively.