Remove two unused variables, causing compiler warnings.

src/video/x11/SDL_x11events.c

@@ -560,9 +560,7 @@ X11_DispatchEvent(_THIS)
                    xevent.xconfigure.width, xevent.xconfigure.height);
 #endif
             long border_left = 0;
-            long border_right = 0;
             long border_top = 0;
-            long border_bottom = 0;
             if (data->xwindow) {
                 Atom _net_frame_extents = X11_XInternAtom(display, "_NET_FRAME_EXTENTS", 0);
                 Atom type = None;
@@ -577,9 +575,7 @@ X11_DispatchEvent(_THIS)
                 if (type != None && nitems == 4)
                 {
                     border_left = ((long*)property)[0];
-                    border_right = ((long*)property)[1];
                     border_top = ((long*)property)[2];
-                    border_bottom = ((long*)property)[3];
                 }
             }