[PA][BRP] Remove enable_runtime_backup_ref_ptr_control

It significantly complicated code, and in reality we always set it to
true whenever use_backup_ref_ptr was true.

Bug: 1240054
Change-Id: I465a24efa9e33ef11402bd6f62f3eab2e24a1586
Cq-Do-Not-Cancel-Tryjobs: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3120906


Reviewed-by: Keishi Hattori <keishi@chromium.org>
Reviewed-by: Garrett Beaty <gbeaty@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Will Harris <wfh@chromium.org>
Commit-Queue: Bartek Nowierski <bartekn@chromium.org>
Cr-Commit-Position: refs/heads/main@{#915928}