staging: line6: drop unused CONFIG_LINE6_USB_DEBUG

The CONFIG_LINE6_USB_DEBUG option is no longer relevant since dyndbg
dev_dbg() is now used instead of a compile-time decision whether to
enable debug messages or not.

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