mustek_usb: Fix [-Wunused-const-variable=] compiler warning

The brightness option that used the char_range variable was renamed to
threshold and changed to use u8_range in c95e4638.

backend/mustek_usb.c

@@ -100,10 +100,6 @@ static SANE_Int new_dev_alloced;
 
 static SANE_String_Const mode_list[6];
 
-static const SANE_Range char_range = {
-  -127, 127, 1
-};
-
 static const SANE_Range u8_range = {
   0,				/* minimum */
   255,				/* maximum */