Remove unused interstitial strings and remove NEW suffix

Now that the red interstitial facelift feature has been launched
successfully on Stable since November 30th, we will remove the old
strings and remove the "_NEW" suffix from the latest string names.

Bug: b/294232396
Change-Id: Iaf065ee0f0aae2b42beb312a10eef0c15c9ad64e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5425810


Commit-Queue: Sarah Krakowiak <skrakowi@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Reviewed-by: Nate Fischer <ntfschr@chromium.org>
Reviewed-by: Matt Mueller <mattm@chromium.org>
Reviewed-by: Daniel Rubery <drubery@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1283381}