staging: line6: delete an unused function

line6_send_sysex_message_async() isn't called from anywhere.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Stefan Hajnoczi <stefanha@gmail.com>
Reviewed-by: Johannes Thumshirn <morbidrsa@googlemail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>