[clang-tidy] remove pointless void arg

Found with modernize-redundant-void-arg

Signed-off-by: Rosen Penev <rosenp@gmail.com>

src/output/plugins/JackOutputPlugin.cxx

@@ -419,7 +419,7 @@ JackOutput::Connect()
 }
 
 static bool
-mpd_jack_test_default_device(void)
+mpd_jack_test_default_device()
 {
 	return true;
 }

src/output/plugins/PulseOutputPlugin.cxx

@@ -872,7 +872,7 @@ try {
 }
 
 static bool
-pulse_output_test_default_device(void)
+pulse_output_test_default_device()
 {
 	return PulseOutput::TestDefaultDevice();
 }