MAINTAINERS: remove drivers/block/ub.c

This driver was removed by commit 68a5059e (block: remove the deprecated
ub driver).

Cc: Pete Zaitcev <zaitcev@redhat.com>
Cc: linux-usb@vger.kernel.org
Signed-off-by: Cesar Eduardo Barros <cesarb@cesarb.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>