Input: kill maple_keyb.c driver

The bus for this was removed entirely some time ago, as well as most
of the drivers that referenced it. maple_keyb seems to have been the
odd one out, and was still sitting in the source tree (though not
actually part of the build system). Kill off the rest of it..

Signed-off-by: Paul Mundt <lethal@linux-sh.org>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>