output_all: explicitly return "true" from audio_output_all_play()

Instead of returning the local variable "ret" which is always true at
this point, hard-code the "true" return value, because that might be
more readable.

src/output_all.c

@@ -240,7 +240,7 @@ audio_output_all_play(struct music_chunk *chunk)
 		if (audio_output_is_open(&audio_outputs[i]))
 			audio_output_play(&audio_outputs[i]);
 
-	return ret;
+	return true;
 }
 
 bool