Remove features::kGainmapHdrImages

This always on now.

Bug: 40284413
Change-Id: I8c7bbbaba3ce58140537b24d0fb5d99cf23cae68
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5826561


Commit-Queue: ccameron chromium <ccameron@chromium.org>
Reviewed-by: Wan-Teh Chang <wtc@google.com>
Reviewed-by: Fernando Serboncini <fserb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1349543}