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: I8ced8050572b7d18a51f05891ebac302818d5ca9