[ntp-ct] After-launch feature cleanup

- Remove usages of the already launched NtpComprehensiveTheming flag.
- Updates expiry milestone for NtpComprehensiveThemeRealbox flag.

Bug: 1375876
Change-Id: Ic1d5e2540a6fc5e6179e9c1d6c4bfe46dbf7c780
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4549770


Reviewed-by: Thomas Lukaszewicz <tluk@chromium.org>
Commit-Queue: Roman Arora <romanarora@chromium.org>
Code-Coverage: Findit <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Riley Tatum <rtatum@google.com>
Cr-Commit-Position: refs/heads/main@{#1147337}