Staging: quatech_usb2: remove unused qt2_box_flush function

In today linux-next I got a compilation warning:

drivers/staging/quatech_usb2/quatech_usb2.c:1852: warning:
‘qt2_box_flush’ defined but not used

Signed-off-by: Javier Martinez Canillas <martinez.javier@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>