ahc(4) clean up old Linux defines

Linux removed theirs starting in 2018 in commit:
"scsi: aic7xxx: Fix build using bare-metal toolchain"

Also remove now-useless sys/cdefs.h includes

Signed-off-by: HP van Braam <hp@tmm.cx>
Reviewed by: imp, mav, emaste
Pull Request: https://github.com/freebsd/freebsd-src/pull/1189