[SH] Remove Safety Check from settings

https://crrev.com/c/6700791 was supposed to remove SafetyHub references
to the flag and as such clean up as much as possible of the old
SafetyCheck code. It forgot to delete the SafetyCheck preference from
the main preferences xml.

Fixed: 439713002
Bug: 383479015
Change-Id: Ia75c7b2c4b191ad8f8976eca29376bb3314fd914
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6863014
Reviewed-by: Gazal Agarwal <agazal@google.com>
Reviewed-by: Theresa Sullivan <twellington@chromium.org>
Commit-Queue: Filipa Senra <fsenra@google.com>
Cr-Commit-Position: refs/heads/main@{#1503795}