[iOS] Remove unused NTP code

This CL removes the NTP code used in the pre-UIRefresh UI.

Bug: 761817
Change-Id: I0d861b77c342d929bf12b8855d9b3d5b060ae826
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1731842
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Auto-Submit: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#683594}