examples/audio/04-multiple-streams: Remove drag-and-drop support.

I'm going to reuse that code for an actual drag/drop example later, but for
simplicity and accessibility of the examples, this is just going to load two
wavs and loop them, so you get the music with a sword-clinking sound mixed
over it.

test/LICENSE.txt

@@ -34,6 +34,14 @@ Thanks to Will for permitting us to distribute this sample with SDL!
 
 
 
+This directory contains sword.wav:
+
+  sword04.wav by Erdie
+  Original: https://freesound.org/s/27858/
+  License: https://creativecommons.org/licenses/by/4.0/
+
+
+
 The .bmp files used by testaudio.c were made by AlDraw:
 
 https://linktr.ee/AlexDraw