[iOS] Remove NewTabPageOmniboxPositioning protocol

NewTabPageOmniboxPositioning is no longer being used by any class
aside from the one which implements it, NewTabPageViewController.

Change-Id: Ia5059eece92fd95d4009359522db6bed58a5cc61
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4481800
Commit-Queue: Sergio Collazos <sczs@chromium.org>
Commit-Queue: Adam Arcaro <adamta@google.com>
Reviewed-by: Sergio Collazos <sczs@chromium.org>
Auto-Submit: Adam Arcaro <adamta@google.com>
Cr-Commit-Position: refs/heads/main@{#1136824}