Remove the redundant ipfilter IPv6 rc rules load.

As of ipfilter 5.1.2 the IPv4 and IPv6 rules tables have been merged.
The ipf(8) -6 option has been a NOP since then. Currently the additional
ipf -6 load statement in rc.d/ipfilter simply added the second ipfilter
rules file to the table already populated by the previous ipf command.
Plenty of time has passed since ipfilter 5.1.2 was imported. It is time to
remove the option from rc.conf and the rc script.

Differential Revision: https://reviews.freebsd.org/D28615