app: remove excess call to Start callback

Change-Id: Ida06858336eb482d1d804279c468e09fad12b97f
Reviewed-on: https://go-review.googlesource.com/5578
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

app/loop_android.go

@@ -104,12 +104,6 @@ func windowDrawLoop(cb Callbacks, w *C.ANativeWindow, queue *C.AInputQueue) {
 	// Wait until geometry and GL is initialized before cb.Start.
 	runStart(cb)
 
-	// We start here rather than onStart so the window exists and the Gl
-	// context is configured.
-	if cb.Start != nil {
-		cb.Start()
-	}
-
 	for {
 		processEvents(cb, queue)
 		select {