xen/mcelog: drop __MC_MSR_MCGCAP

It has never been part of Xen's public interface, and there's therefore
no guarantee for MCG_CAP's value to always be present in array entry 0.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Juergen Gross <jgross@suse.com>