Remove unused stored extension in Decoder

As preparation for issue #1487

src/modules/sound/Decoder.h

@@ -41,7 +41,7 @@ public:
 
 	static love::Type type;
 
-	Decoder(Data *data, const std::string &ext, int bufferSize);
+	Decoder(Data *data, int bufferSize);
 	virtual ~Decoder();
 
 	/**
@@ -149,9 +149,6 @@ protected:
 	// reads in the future.
 	StrongRef<Data> data;
 
-	// File extension.
-	std::string ext;
-
 	// When the decoder decodes data incrementally, it writes
 	// this many bytes at a time (at most).
 	int bufferSize;