[Android] Remove PlusAddressAndroidSettingsEntry flag.

This CL incorporates the plus address settings entry under the
kPlusAddressesEnabled feature flag, because the settings menu entry
must be visible only if the base feature is enabled.  The original
settings entry flag is removed.

Bug: 363733915
Change-Id: I3bdde3488fa3acd1688135b1ae5d7d2b7615e8ef
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5942371
Reviewed-by: Jan Keitel <jkeitel@google.com>
Commit-Queue: Timofey Chudakov <tchudakov@google.com>
Cr-Commit-Position: refs/heads/main@{#1371306}