Remove SimpleMenuModel as forward declaration

This CL removes an unnecessary forward declaration, since there is an
include for the class we just don't need it.

Bug: None.
Test: Build.
Change-Id: Ie3b4a940569722f7506c3f4df7316ed0891b5384
Reviewed-on: https://chromium-review.googlesource.com/c/1385384Reviewed-by: Yusuke Sato <yusukes@chromium.org>
Commit-Queue: David Jacobo <djacobo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#618415}

chrome/browser/chromeos/arc/intent_helper/open_with_menu.h

@@ -22,10 +22,6 @@ namespace content {
 class BrowserContext;
 }
 
-namespace ui {
-class SimpleMenuModel;
-}
-
 namespace arc {
 
 // An observer class which populates the "Open with <app>" menu items either