[iOS] Merge grouping feature flag

- Merge with the grouping flag Viktor created in another CL.
- Remove the unused kEnableGroupingForPasswordManager flag.

Bug: 3871998
Change-Id: I5d88845999708f3b6a281cb776cec0f06961a8ea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3891692


Reviewed-by: Mohamed Amir Yosef <mamir@chromium.org>
Commit-Queue: Veronique Nguyen <veronguyen@google.com>
Reviewed-by: Viktor Semeniuk <vsemeniuk@google.com>
Cr-Commit-Position: refs/heads/main@{#1045873}