x86, MCE: Kill CPU_POST_DEAD

In conjunction with cleaning up CPU hotplug, we want to get rid of
CPU_POST_DEAD. Kill this instance here and rediscover CMCI banks at the
end of CPU_DEAD.

Link: http://lkml.kernel.org/r/http://lkml.kernel.org/r/1400750624-19238-1-git-send-email-bp@alien8.de
Signed-off-by: Borislav Petkov <bp@suse.de>