x86, MCE: Use notifier chain only for MCE decoding

Drop the edac_mce custom hook in favor of the generic notifier
mechanism. Also, do not log the error to mcelog if the notified agent
was able to decode it.

Signed-off-by: Borislav Petkov <borislav.petkov@amd.com>
Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>