Remove automatic IKE version negoiation.

According to the strongSwan 4.5.x documentation, the keyexchange configuration value "ike" is a synonym to "ikev2".

In strongSwan 5.0.0 however, the configuration value "ike" will try to negoiate IKEv2 connections but will accept IKEv1 connections if the remote peer sends an IKEv1 request.

The previous behavior where IKEv1 will be unconditionally set is not affected by this change.