p13n: remove screen-saver-duration flag

The feature has launched since M121 and now the flag is expiring soon.

BUG=b:330369169
TEST=ash_unittests --gtest_filter="*WakeLock*"

Change-Id: I6dcf5befc1a4fff2b1cf4a09de6a921d51dd12ce
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5406458


Reviewed-by: Thuong Phan <thuongphan@chromium.org>
Commit-Queue: Jerry Liu <pzliu@google.com>
Cr-Commit-Position: refs/heads/main@{#1280487}