test: remove testaudio-art.txt (this info is in test/LICENSE.txt now).

test/testaudio-art.txt

@@ -1,8 +0,0 @@
-The .bmp files used by testaudio.c were made by AlDraw:
-
-https://linktr.ee/AlexDraw
-
-They may be distributed as public domain files!
-
-
-