Use the shared Music Pack for main menu music

manifest.json

@@ -8,7 +8,7 @@
   "sources": [
     "src/**/*.cs"
   ],
-  "depends": ["ui_base>=1"],
+  "depends": ["ui_base>=1", "music_pack>=1"],
   "test_suites": [
     {"id": "menu_flow", "activate": ["--itest=menu_flow"]},
     {"id": "save_dormancy", "activate": ["--itest=save_dormancy"]},

music/playlists/mainmenu.json

@@ -1,11 +0,0 @@
-{
-  "Scene": "MainMenu",
-  "Musics": [
-    {
-      "MusicType": "Ambient",
-      "MusicNames": [
-        "Serpent's Lament"
-      ]
-    }
-  ]
-}