usbvision: remove some unused vars

Gcc 6.1 warns about some unused vars. Remove them:

drivers/media/usb/usbvision/usbvision-core.c:94:18: warning: 'min_imgheight' defined but not used [-Wunused-const-variable=]
 static const int min_imgheight = MIN_FRAME_HEIGHT;
                  ^~~~~~~~~~~~~
drivers/media/usb/usbvision/usbvision-core.c:93:18: warning: 'min_imgwidth' defined but not used [-Wunused-const-variable=]
 static const int min_imgwidth = MIN_FRAME_WIDTH;
                  ^~~~~~~~~~~~
drivers/media/usb/usbvision/usbvision-core.c:92:18: warning: 'max_imgheight' defined but not used [-Wunused-const-variable=]
 static const int max_imgheight = MAX_FRAME_HEIGHT;
                  ^~~~~~~~~~~~~
drivers/media/usb/usbvision/usbvision-core.c:91:18: warning: 'max_imgwidth' defined but not used [-Wunused-const-variable=]
 static const int max_imgwidth = MAX_FRAME_WIDTH;
                  ^~~~~~~~~~~~
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

drivers/media/usb/usbvision/usbvision-core.c

@@ -88,11 +88,6 @@ MODULE_PARM_DESC(adjust_y_offset, "adjust Y offset display [core]");
 #define DBG_SCRATCH	(1 << 4)
 #define DBG_FUNC	(1 << 5)
 
-static const int max_imgwidth = MAX_FRAME_WIDTH;
-static const int max_imgheight = MAX_FRAME_HEIGHT;
-static const int min_imgwidth = MIN_FRAME_WIDTH;
-static const int min_imgheight = MIN_FRAME_HEIGHT;
-
 /* The value of 'scratch_buf_size' affects quality of the picture
  * in many ways. Shorter buffers may cause loss of data when client
  * is too slow. Larger buffers are memory-consuming and take longer