fixes a segfault on relayd grammar, when using the "disable" keyword in
a router section inside a realyd.conf. disabling routers by relayd.conf
or run-time is not not documented in relayd.conf(5) or relayctl(8),
neither the necessary code to make such a functionality works exists.
this removes "disable" keyword grammar processing for routers, therefore
keeping consistency with manpages, also removes the only bit in pfe.c
where F_DISABLE flag is checked (but never set before though) in a
router structure which happens for pfctl-show purposes only.