decoder: removed plugin method try_decode()

Instead of having a seprate try_decode() method, let the
stream_decode() and file_decode() methods decide whether they are able
to decode the song.

src/decoder_api.h

@@ -60,12 +60,6 @@ struct decoder_plugin {
 	 */
 	void (*finish)(void);
 
-	/**
-	 * returns true if the input stream is decodable by the
-	 * decoder plugin, false if not
-	 */
-	bool (*try_decode)(struct input_stream *);
-
 	/**
 	 * this will be used to decode InputStreams, and is
 	 * recommended for files and networked (HTTP) connections.