Remove UMS support code from radeonkms.

The code is unreachable since the entries of radeon_ioctls[] are not
associated with any device: we provide only the KMS entry points.
Moreover, r600_cp_dispatch_texture() contains an integer overflow bug
that can be triggered from userspace.[1]

Reported by: Anonymous of the Shellphish Grill Team[1]
Reviewed by: dumbbell
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D18516