Remove OEM_UNLOCK_PROP usage

 * It was only used for adb host scripts, but after commit
   "Remove sys.oem_unlock_allowed check for Android device"
   in 2016, there is no usage for it. Remove it because this prop
   is now exposed to untrusted apps, which is not desirable.

Change-Id: Ic1c2f4e06b3a07ecf3f724fe5085974c626cb3fc