P2P: Remove Label config method

P2P: Remove Label config method

The P2P specification (3.1.4.3) disallows use of the Label configuration
method between two P2P devices. This was previously enforced at upper
level, but the obsolete code can be removed from wpa_supplicant. This
adds a bit more strict enforcement of the policy, but should not result
in practical differences since no known P2P implementation uses Label
config method.