MAINTAINERS: remove drivers/char/hpet.c

bob.picco@hp.com doesn't work any more and Bob says that he's unlikely to
work on hpet.c in the future.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Acked-by: Bob Picco <bpicco@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>