Delete AppsSyncOptinFileWriteAttempts histogram.

Apps Sync optin file has been working well in production for a long time
now, no longer a need to track this.

OBSOLETE_HISTOGRAM[Cros.AppsSyncOptinFileWriteAttempts]=no longer tracked or needed

Bug: b:41485392
Change-Id: I4db42ea24489381ea585755a29dc849bda49bb0c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5281169
Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Miriam Zimmerman <mutexlox@chromium.org>
Reviewed-by: Nigel Tao <nigeltao@chromium.org>
Reviewed-by: David Munro <davidmunro@google.com>
Commit-Queue: maciek swiech <drmasquatch@google.com>
Cr-Commit-Position: refs/heads/main@{#1260525}