Staging: comedi: remove unused functions from comedilib.h

Remove the functions that are not used from this file.  Now it will
be easier to determine what code can be removed from kcomedilib by
using sparse.

Cc: Ian Abbott <abbotti@mev.co.uk>
Cc: Frank Mori Hess <fmhess@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>