Refactor: various: Guard is_corosync_cluster() with SUPPORT_COROSYNC

Mostly to be consistent. Efficiency boost is negligible.

Also remove two redundant calls, where we've already checked the return
value of get_cluster_type().

Remove is_corosync_cluster() check completely for one case where the
contents aren't corosync-specific.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>