Remove snd_aureal driver source

This driver was not finished when it was committed in 1999 and was never
connected to the build.

A version of the driver used to be available in ports as
audio/aureal-kmod, but it has been removed. It did not build on FreeBSD
10.x or later and the binary objects it required were not available
after Google Code disappeared.

PR: 124343
Reported by: joel
MFC after: 1 week
Sponsored by: The FreeBSD Foundation

(cherry picked from commit ef5dec7cc75b985d9ada767f716e19ec44f5c44a)