[ios] Remove unused import from tab_group.proto

Bug: none
Change-Id: I3f3e7a45535dad1d610778763c708dfc78b6f6dc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5402148


Reviewed-by: Ewann Pellé <ewannpv@chromium.org>
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1280120}