MAINTAINERS: update kdump maintainers

Remove myself from kdump maintainers as I have no enough time to maintain
it now.  But I can review patches on demand though.

Link: https://lkml.kernel.org/r/YZyKilzKFsWJYdgn@dhcp-128-65.nay.redhat.com


Signed-off-by: Dave Young <dyoung@redhat.com>
Acked-by: Baoquan He <bhe@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>