staging: line6: drop monitor_level sysfs attr

The monitor level can be adjusted using an ALSA volume control.
Therefore the sysfs attribute is unnecessary.  Note that we leave the
monitor_level driver variable so that the ALSA volume control still
works.

Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>