vo_opengl: remove leftover variable from vaglx in vaegl

This was preventing compilation on systems without X11 headers.

video/out/opengl/hwdec_vaegl.c

@@ -88,7 +88,6 @@ struct priv {
     struct mp_log *log;
     struct mp_vaapi_ctx *ctx;
     VADisplay *display;
-    Display *xdisplay;
     GLuint gl_textures[4];
     EGLImageKHR images[4];
     VAImage current_image;