[ios] Remove GradientView Feature code

Change-Id: Iefe303af05e8024195b77466e717011d1d1b34be
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5617440


Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Commit-Queue: Scott Yoder <scottyoder@google.com>
Auto-Submit: Chris Lu <thegreenfrog@chromium.org>
Reviewed-by: Scott Yoder <scottyoder@google.com>
Cr-Commit-Position: refs/heads/main@{#1313048}