Bluetooth: Don't set a null alias before unpairing

* This causes the unpair action to throw a null pointer exception and
  bring down the Settings app. Don't do this.

Change-Id: I0c60437f5ea934385a037a464743e24c9d341aca
(cherry picked from commit df1f9797eb380f9d1acbfd0967b66929f5b35466)