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Clean up nicknameManagementEnabled flag and remove the legacy elements and
CSS config.

Local build test video after clean up(googlers only):
https://drive.google.com/file/d/1uYmVuIjlThiGl-TTeGvPrZWvDtkBJwKJ/view?usp=sharing&resourcekey=0-hMW3nVLEyQ7fbLzV5RHwqg

Bug: 1082013
Change-Id: Ie5ba1372cf510decb17a113c67ed5aee11eeb755
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2600180
Commit-Queue: Sujie Zhu <sujiezhu@google.com>
Reviewed-by: Esmael Elmoslimany <aee@chromium.org>
Reviewed-by: Jared Saul <jsaul@google.com>
Cr-Commit-Position: refs/heads/master@{#839145}