fastd: Allow IPv6 remotes for peers

The "ipv4" option is used to enforce using ipv4 when
using dns for fastd remotes.
However this option was incorrectly always set which
makes ipv6 connections impossible.
Because enforcing ipv4 is not necessary, this option
is removed.

Signed-off-by: Fabian Bläse <fabian@blaese.de>
Reviewed-by: Christian Dresel <fff@chrisi01.de>
Reviewed-by: Tim Niemeyer <tim@tn-x.org>
Tested-by: Fabian Bläse <fabian@blaese.de>