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.