The {offset_reg} parameter was introduced so that the signature of
StoreTaggedPointer is the same as for Store, and also to prepare for
potential later uses. However, handling the parameter would introduce
additional complexity, and chances are low that it will ever be used.
That's why this CL just removes it.

R=thibaudm@chromium.org

Bug: v8:7581
Change-Id: Ib419a72e1d9f36b8d9a3dbbce538ce57bb37d476
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2412182Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69958}