an: Deprecate the driver

Last an(4) devices have been End Of Life and End Of Sale in 2007.
Time to remove this driver.

Differential Revision: https://reviews.freebsd.org/D30678
Reviewed by: imp (earlier version), adrian (earlier version)
MFC after: 3 days
Sponsored by: Diablotin Systems

(cherry picked from commit ab30bb8270fa99503a0f204543a9ac3e3c7fa04d)