Accessibility Service & Shortcut Redesign - Remove “volume key shortcut” setting (5/n)

- Target to fix migration issue to support a user revoked the accessibility shortcut before upgrading to R, but wants to to apply new hardware shortcut design in R.
- The switchbar of accessibility_shortcut_enabled was removed in (1/n), so we should also remove related checker in controller.

Bug: 142529032
Test: atest AccessibilityShortcutControllerTest
Change-Id: I48f8d88c4192e894b9e210221e5dc7e701344623