[omnibox] Drop support  for non-substituting keywords in the Omnibox.

This CL updates KeywordProvider to drop support for non-substituting
keywords in the Omnibox (match type = HISTORY_KEYWORD), which were not
intended to be a major supported use-case of keywords in the first
place.

Users who used to rely on this functionality (i.e. non-substituting
keywords) can make use of bookmarks to achieve the same kind of effect
that was once served by such keywords. For example, instead of defining
a non-substituting keyword "foo" that navigates to "foo.com", users can
simply bookmark the "foo.com" webpage and title the bookmark "foo".
Then, entering "foo" in the Omnibox will surface the relevant
suggestion via BookmarkProvider.

Bug: 388070061
Change-Id: I8048381e9b0343da0dd1f8059f46b19229d55167
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6164610
Reviewed-by: manuk hovanesian <manukh@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Commit-Queue: Khalid Peer <khalidpeer@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1411850}