Remove Meta+Backspace=Back shortcut properly

Previously this key combination was removed from the user facing
elements like the "new shortcut helper" UI but wasn't removed from
old shortcut helper and the key gesture controller.

Originally removed due to low usage and ergonomic difficulty with
the shortcut: b/394599430

Bug: 394599430
Bug: 394600915
Test: atest InputTests
Flag: EXEMPT bugfix
Change-Id: I2968c76bea7fe9d85c2b6f08d7277933f5ee35db