Fix Theora/Webm loaders declaring binary extension

RES_BASE_EXTENSION is only to declare our proprietary binary formats
(e.g. "scn", "res", etc.).

Fixes #20395.

modules/webm/video_stream_webm.h

@@ -109,7 +109,6 @@ private:
 class VideoStreamWebm : public VideoStream {
 
 	GDCLASS(VideoStreamWebm, VideoStream);
-	RES_BASE_EXTENSION("webm");
 
 	String file;
 	int audio_track;