MAINTAINERS: remove unnecessary linux-kernel@vger.kernel.org entries

CC'ing lkml is the default and doesn't need separate entries.

Signed-off-by: Joe Perches <joe@perches.com>
Acked-by: Harry Wei <harryxiyou@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>