Remove the dark mode build flag and merge res_night/ into normal res/
directory. Also removes the dark mode flag from about://flags.

BUG=936275

Change-Id: Icd8df11f94ef9ab6314c4a2fcd47ecab261985a9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2031877
Commit-Queue: Theresa  <twellington@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Peter Wen <wnwen@chromium.org>
Reviewed-by: Sinan Sahin <sinansahin@google.com>
Cr-Commit-Position: refs/heads/master@{#739206}