Remove kill switch for checking request_file_set_ in CanRequestURL().

The features::kRequestFileSetCheckedInCanRequestURL kill switch was
introduced in https://crrev.com/c/5005884 to guard the move of
request_file_set_ checks from CanCommitURL() to CanRequestURL(). The
feature has been enabled since M121, and with stable releases already
being past that milestone, it should be safe to clean this up and
remove the old path.

Bug: 40539942
Change-Id: I194183d641a01e2c4ef8485e8ff24e5c07fd528c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5401455


Reviewed-by: Charlie Reis <creis@chromium.org>
Commit-Queue: Alex Moshchuk <alexmos@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1280003}