staging: comedi: drivers: remove bogus ni_mio_c_common.c

The zero-length file "ni_mio_c_common.c" was inadvertantly created by
commit e563637b ("staging: comedi: Use ARRAY_SIZE for sizes of
arrays").  Remove it.
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>