Oobe: remove EnableBetterAssistant feature

Feature was launched in M91.

Bug: 1219411
Change-Id: I23e71c353dd4f55399f94ef4315a651c4ef46f84
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3103071


Reviewed-by: Roman Sorokin [CET] <rsorokin@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Yue Li <updowndota@chromium.org>
Commit-Queue: Yunke Zhou <yunkez@google.com>
Cr-Commit-Position: refs/heads/main@{#914747}