brew.sh: don't set HOMEBREW_NO_INSTALL_FROM_API` automatically.

My understanding from conversations with Bo98 is that this should now
work fine on both older macOS versions and non-default prefixes.

Similarly, it's super confusing when this is set automatically and you
can't figure out why...

Library/Homebrew/brew.sh

@@ -593,12 +593,6 @@ then
     # shellcheck disable=SC2034
     HOMEBREW_MACOS_SYSTEM_RUBY_NEW_ENOUGH="1"
   fi
-
-  # Don't support API at this time for older macOS versions.
-  if [[ "${HOMEBREW_MACOS_VERSION_NUMERIC}" -lt "${HOMEBREW_MACOS_OLDEST_SUPPORTED_NUMERIC}" ]]
-  then
-    export HOMEBREW_NO_INSTALL_FROM_API=1
-  fi
 else
   HOMEBREW_PRODUCT="${HOMEBREW_SYSTEM}brew"
   [[ -n "${HOMEBREW_LINUX}" ]] && HOMEBREW_OS_VERSION="$(source /etc/os-release && echo "${PRETTY_NAME}")"
@@ -671,12 +665,6 @@ Your Git executable: $(unset git && type -p "${HOMEBREW_GIT}")"
   fi
 fi
 
-# Generic OS or non-default prefix: API not supported.
-if [[ (-z "${HOMEBREW_MACOS}" && -z "${HOMEBREW_LINUX}") || "${HOMEBREW_PREFIX}" != "${HOMEBREW_DEFAULT_PREFIX}" ]]
-then
-  export HOMEBREW_NO_INSTALL_FROM_API=1
-fi
-
 setup_ca_certificates() {
   if [[ -n "${HOMEBREW_FORCE_BREWED_CA_CERTIFICATES}" && -f "${HOMEBREW_PREFIX}/etc/ca-certificates/cert.pem" ]]
   then