Retire obsolete sysctl hw.bus.devctl_disable

hw.bus.devctl_disable has tagged been obsolete for a decade. Remove it. Also
remove some long obsolete comments. This was done and backed out once in 2014,
but we've had enough releases with the 'new' method of setting queue length that
we can just remove this sysctl now (stable/11, stable/12 and current all don't
reference it).