remove unused header file: drivers/video/riva/nv4ref.h

Signed-off-by: Robert P. J. Day <rpjday@mindspring.com>
Cc: "Antonino A. Daplas" <adaplas@pol.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>