staging: line6: Remove unsupported X3 devices

Support for these devices appears to have never been completed.  Remove
them from the device table along with a minimal amount of supporting
code.

Signed-off-by: Chris Rorvick <chris@rorvick.com>
Reviewed-by: Stefan Hajnoczi <stefanha@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>