[TabGroupController updates] Remove unused function MoveTabGroup

MoveTabGroup is only defined and used within the TabStripModel, we can
remove this method from the TabGroupController, and just define the
notification within TabStripModel header as non virtual.

Bug: 417235218
Change-Id: Icd6a36d8879f92d98cf0daa0f76431523628c1e6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6534636
Reviewed-by: Eshwar Stalin <estalin@chromium.org>
Reviewed-by: Shibalik Mohapatra <shibalik@chromium.org>
Commit-Queue: David Pennington <dpenning@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1459800}