network: remove firewalld version check from networkSetBridgeZone()

network: remove firewalld version check from networkSetBridgeZone()

At the time the version check in this function was written, there were
still several supported versions of some distros that were using a
version of firewalld too old to support the "rich rule priorities"
used by the 'libvirt' zone that we installed for firewalld. Today the
newest distro that has a version of firewalld < 0.7.0 is
RHEL7/CentOS7, so we can remove the complexity and if the libvirt zone
is missing simply say "the libvirt zone is missing".

Signed-off-by: Laine Stump <laine@redhat.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>