Avoid using bioq_* in gmirror.

gmirror does not perform any sorting of I/O requests, so the bioq API
doesn't provide any advantages over plain TAILQs. The API also does not
provide operations needed by an upcoming change.

No functional change intended. The diff shrinks the geom_mirror.ko
text and the gmirror softc slightly.

Tested by: pho (part of a larger patch)
MFC after: 1 week
Sponsored by: Dell EMC Isilon