[ios] Remove NamedGuideUtil code

This CL removes NamedGuideUtil code because in current implementation
there is only one GuideName in the NSArray<GuideName*>* (note that this
GuideName will be soon moved to LayoutGuideCenter).

This is an initial cleanup, kContentAreaGuide will be moved in a
follow-up CL.

Bug: 1450600
Change-Id: I94d91f1f40b86e1f0596a6297b95be7a554a9631
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5099043
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Commit-Queue: Federica Germinario <fedegermi@google.com>
Reviewed-by: Asami Doi <asamidoi@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1234919}