Remove stg(4) driver

stg(4) is marked as gone in 12. Remove it. There are no sightings of
it in the nycbug dmesg database. It was for an obscure SCSI card that
sold mostly in Japan, and was especially popilar among pc98 hackers in
the 4.x time frame. It was also only enabled on i386.

Relnote: Yes