Remove main menu sounds for now

Currently there is no way to disable music in main menu which may be annoying to players. Loading the main menu also takes much longer.

menu/README.txt

@@ -1,20 +0,0 @@
-Main Menu
-=========
-
-License of media (textures and sounds)
---------------------------------------
-Attribution-ShareAlike 3.0 Unported (CC BY-SA 3.0)                                 
-http://creativecommons.org/licenses/by-sa/3.0/
-
-Authors of media files
------------------------
-
-Casimir (CC BY-SA 4.0)
-  header.png
-  icon.png
-
-Trevor Lentz (CC BY-SA 3.0)
-https://opengameart.org/users/trevor-lentz
-  theme.1.ogg (Deus Ex Tempus)
-  theme.2.ogg (Pixel River)
-  theme.3.ogg (Melody Eternal)