[ios] Inline trivial bookmark operations and fix names/comments

There is nowadays a single BookmarkModel per BrowserState, so waiting
for them to be loaded as well as other operations can be trivially
achieved without helper functions.

The patch also improves several names and comments to avoid conveying
the idea of multiple BookmarkModel instances.

The patch intends no behavioral differences but it is hard to
guarantee so: the former AreAllAvailableBookmarkModelsLoaded() may
deal with scenarios where BookmarkModel doesn't exist, whereas the
new code expects there is a bookmark model. This assumption should
hold true but it is non-trivial.

Bug: 346918509
Change-Id: I58f97a013ebfd12fc15714cc8b5c5961771ffe9b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5701247
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Jérôme Lebel <jlebel@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1326754}