[Tab Switcher] Remove unused group creation button

Dead code since circa 2019 that I spotted while talking with bjfong@
today.

AFAICT this is currently only used in a testcase and is not accessible
in any production code.

Bug: N/A
Change-Id: Ibb8fd8aadc1fb8bd4759657ebe5c33110d1dba4b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5464326


Commit-Queue: Calder Kitagawa <ckitagawa@chromium.org>
Reviewed-by: Brandon Fong <bjfong@google.com>
Cr-Commit-Position: refs/heads/main@{#1289600}