ath(4): Remove MIPS SoC build glue and AR9130 from FreeBSD HAL

All of these are obsoleted by the general removal of MIPS support.

Actually, corresponding to the removed AH_SUPPORT_x, there is more
superfluous support sprinkled across the HAL source. However, that
code is left in place for now in order to ease a sync to NetBSD.

Reviewed by: emaste (w/ man page fix)
Approved by: adrian
Differential Revision: https://reviews.freebsd.org/D41355