staging: comedi: comedidev.h: remove unused "polling" function prototypes

These "polling" functions are not implemented in the comedi core. Remove the
unnecessary prototypes.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>