staging: comedi: remove attach_pci and attach_usb handlers

No comedi drivers set the `attach_pci()` or `attach_usb()` handlers in
their `struct comedi_driver` any longer as they have all been replaced
with an `auto_attach()` handler.  Also, no code calls the `attach_pci()`
or `attach_usb()` handlers any longer.  Remove them from `struct
comedi_driver`.

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