opus/vorbis: Remove dead code not used since 3.0

Since the new audio system in 3.0 we switched the OGG support to
stb_vorbis, and the Opus stream support was disabled as incompatible
(see #7496).

We still build the libraries as they are needed by the theora and webm
modules, but we don't need any Godot code apart from `register_types`.

Fixes #7496.

modules/ogg/SCsub

@@ -3,6 +3,9 @@
 Import('env')
 Import('env_modules')
 
+# Only kept to build the thirdparty library used by the theora and webm
+# modules.
+
 env_ogg = env_modules.Clone()
 
 # Thirdparty source files