[iOS] Remove hidden feed view

With the launch of home customization (and the removal of the
`feedExpandedPref`), there won't be a state when the feed should have
its header visible but content hidden.

That means we can remove the "views for hidden feed" from the feed
header view, as well as their dependencies. This also removes the need
for the control delegate's `shouldFeedBeVisible` method. We might plug
this delegate to the feed service in the future so the service could
toggle feed visibility state, so removing this getter method might
prevent potential risk of circular reference.

Bug: N/A
Change-Id: I0cc7c15339e63e9ce1ce110c075e958698c0d8d8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6487807
Reviewed-by: Adam Arcaro <adamta@google.com>
Auto-Submit: Ginny Huang <ginnyhuang@chromium.org>
Commit-Queue: Adam Arcaro <adamta@google.com>
Cr-Commit-Position: refs/heads/main@{#1451446}