vout: do not set "fullscreen" variable again (fixes #7926)

The "fullscreen" variable is set by the UI. That triggers a vout
control request. Then the vout thread actually switches to/from
full screen mode. Setting the "fullscreen" variable again then would
trigger a live loop in some circumstances, and should be useless.

src/video_output/video_output.c

@@ -1268,8 +1268,6 @@ static void ThreadStep(vout_thread_t *vout, mtime_t *duration)
 
 static void ThreadChangeFullscreen(vout_thread_t *vout, bool fullscreen)
 {
-    /* FIXME not sure setting "fullscreen" is good ... */
-    var_SetBool(vout, "fullscreen", fullscreen);
     vout_SetDisplayFullscreen(vout->p->display.vd, fullscreen);
 }