Mash cleanup: message center part 1.

Remove some methods from AshMessageCenterController/Client. These are
the methods used to show and interact with notifications. The
notifications can be posted directly from Chrome to the message center
since there's only one process.

TBR=tsepez@chromium.org

Bug: 958191
Change-Id: I4d64e5105c8428a2022f9fd86d349b350514d49f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1652358
Reviewed-by: Evan Stade <estade@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#668118}