output/Thread: move software_mixer_set_filter() call to Open()

.. and remove the obsolete method OpenFilter().

src/output/Internal.hxx

@@ -434,13 +434,6 @@ private:
 	 */
 	void CloseOutput(bool drain);
 
-	/**
-	 * Throws std::runtime_error on error.
-	 *
-	 * Mutex must not be locked.
-	 */
-	AudioFormat OpenFilter(AudioFormat &format);
-
 	/**
 	 * Mutex must not be locked.
 	 */

src/output/OutputThread.cxx

@@ -89,22 +89,6 @@ AudioOutput::Disable()
 	}
 }
 
-inline AudioFormat
-AudioOutput::OpenFilter(AudioFormat &format)
-{
-	assert(format.IsValid());
-
-	const auto result = source.Open(format, *request.pipe,
-					prepared_replay_gain_filter,
-					prepared_other_replay_gain_filter,
-					prepared_filter);
-
-	if (mixer != nullptr && mixer->IsPlugin(software_mixer_plugin))
-		software_mixer_set_filter(*mixer, volume_filter.Get());
-
-	return result;
-}
-
 void
 AudioOutput::CloseFilter()
 {
@@ -136,6 +120,9 @@ AudioOutput::Open()
 				prepared_other_replay_gain_filter,
 				prepared_filter)
 			.WithMask(config_audio_format);
+
+		if (mixer != nullptr && mixer->IsPlugin(software_mixer_plugin))
+			software_mixer_set_filter(*mixer, volume_filter.Get());
 	} catch (const std::runtime_error &e) {
 		FormatError(e, "Failed to open filter for \"%s\" [%s]",
 			    name, plugin.name);