fixes a segfault on relayd grammar, when using disable keyword in a realyd.conf
router section. disabling routers through 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 active exists. so, 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.