Remove 'Allow custom buttons' checkbox (1/2)

Let hardware buttons be rebound by default.

Change-Id: Ib4fcae7d93dc63d53490442d7ff44678583b0d9c
Merge "Remove 'Allow custom buttons' checkbox (1/2)" into cm-10.2
Merge remote-tracking branch 'upstream/cm-10.2' into cm-10.2