[Android][Autofill] Delete keyboard accessory flag.

Keyboard accessory has been enabled for all users on stable.
Remove corresponding feature flag as a clean up step.

Bug: 1448820
Change-Id: Id3cbe21230d72d81c7f4ab99a6ba3a8a53e8243c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4567644


Commit-Queue: Timofey Chudakov <tchudakov@google.com>
Reviewed-by: Friedrich Horschig <fhorschig@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1149133}