mash cleanup: Merge shelf models

Chrome and Ash synchronized separate ShelfModel instances for Mash.
This is no longer necessary, given the discontinuation of Mash.

Remove Chrome's ShelfModel instance, use Ash's instance directly.
Remove shelf.mojom, struct traits, RemoteShelfItemDelegate, etc.

Cleanup ShelfController, ChromeLauncherController, ShelfItemDelegate.
Move ShelfModel access from Shell to a static singleton accessor.
Discontinue usage of menu.mojom for the Shelf; use SimpleMenuModel.
Remove unused shelf_alignment_menu.cc.

TBR=tsepez@chromium.org

Bug: 958215, 958247
Test: Automated; no Chrome OS shelf regressions.
Change-Id: Ib315c596bf1048e6aab3cfb754b9f6b402b93794
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1595553
Commit-Queue: Michael Wasserman <msw@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#660685}

diff --git a/ash/shelf/shelf_window_watcher_item_delegate.h b/ash/shelf/shelf_window_watcher_item_delegate.h
index 4f06c3b9..af9b79a 100644
--- a/ash/shelf/shelf_window_watcher_item_delegate.h
+++ b/ash/shelf/shelf_window_watcher_item_delegate.h

@@ -27,8 +27,8 @@
                     int64_t display_id,
                     ShelfLaunchSource source,
                     ItemSelectedCallback callback) override;
-  void GetContextMenuItems(int64_t display_id,
-                           GetContextMenuItemsCallback callback) override;
+  void GetContextMenu(int64_t display_id,
+                      GetContextMenuCallback callback) override;
   void ExecuteCommand(bool from_context_menu,
                       int64_t command_id,
                       int32_t event_flags,