x86: Remove redundant K6 MSRs

MSR_K6_EFER is unused, and MSR_K6_STAR is redundant with MSR_STAR.

Signed-off-by: Brian Gerst <brgerst@gmail.com>
LKML-Reference: <1279371808-24804-1-git-send-email-brgerst@gmail.com>
Reviewed-by: Pekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>