[ios] Remove remainder of contextual search.

This CL removes the last parts of contextual search from shipping code.

The settings UI is moved into contextual_search/, and is still built
and tested for unit tests, but all of the remaining logic around 
initializing CS and enabling it, and possibly showing the settings,
is removed.

The IDC_SHOW_CONTEXTUAL_SEARCH_SETTINGS command is also removed.

Bug: 738881
Change-Id: I41226091de512660b5969623083c6a450d86cd59
Reviewed-on: https://chromium-review.googlesource.com/577870


Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Commit-Queue: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488331}