[iOS] Remove DarkModeDynamicColor as iOS 12 cleanup

All the uses of DarkModeDynamic color should be gone, and it was only
necessary on iOS 12. Also, I forgot to remove the import statements when
cleaning up everywhere, so this CL removes all of those as well.

Bug: 981889
Change-Id: I581ec86fdfb9477ff2583ad271ec082dc6bb0a77
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2982798
Commit-Queue: Robbie Gibson <rkgibson@google.com>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#896051}