LineageSettingsActivity: Remove useless check for navbar toggle

 * The only condition for "Use on screen navigation keys instead
   of hardware keys" option to be shown is the availability
   of KeyDisabler feature. There is really no need to check if the
   device "needs" a software navbar (because it has no hardware keys).

Change-Id: Id716d766927f440292a6d18d8fa77bed76ad90b3