staging: comedi: remove unused command callback support

The 'kcomedilib' module used to provide functions to allow asynchronous
comedi commands to be set up from another kernel module, but now
commands can only be set up by ioctls from user space via the core
comedi module.  Since support for commands initiated from kernel space
has been dropped, the `cb_func` and `cb_arg` members of `struct
comedi_async` are never set (although the `cb_mask` member is still used
to mask comedi events).  The `SRF_USER` bit of the comedi subdevice
runflags is no longer needed to distinguish commands from user and
kernel space since they only come from user space.

Don't bother setting or testing the `SRF_USER` flag, and get rid of it,
along with the `cb_func` and `cb_arg` members.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>