hw/display/vga:Remove redundant statement in vga_draw_graphic()

Clang static code analyzer show warning:
hw/display/vga.c:1677:9: warning: Value stored to 'update' is never read
        update = full_update;
        ^        ~~~~~~~~~~~

Reported-by: Euler Robot <euler.robot@huawei.com>
Signed-off-by: Chen Qun <kuhn.chenqun@huawei.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Li Qiang <liq3ea@gmail.com>
Message-Id: <20200827110311.164316-11-kuhn.chenqun@huawei.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>

hw/display/vga.c

@@ -1674,7 +1674,6 @@ static void vga_draw_graphic(VGACommonState *s, int full_update)
         if (!(s->cr[VGA_CRTC_MODE] & 2)) {
             addr = (addr & ~0x8000) | ((y1 & 2) << 14);
         }
-        update = full_update;
         page0 = addr & s->vbe_size_mask;
         page1 = (addr + bwidth - 1) & s->vbe_size_mask;
         if (full_update) {