staging: comedi: remove comedi_clear_subdevice_minor()

Merge the code from `comedi_clear_subdevice_minor()` into
`comedi_free_subdevice_minor()` and remove it since nothing else calls
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>