UI: Fix crash when opening transition Properties dialog

Crash was introduced in ae4797f8cc. It seems with that change, this
function call is no longer necessary.

UI/window-basic-properties.cpp

@@ -115,7 +115,6 @@ OBSBasicProperties::OBSBasicProperties(QWidget *parent, OBSSource source_)
 	layout()->addWidget(windowSplitter);
 
 	if (type == OBS_SOURCE_TYPE_TRANSITION) {
-		AddPreviewButton();
 		connect(view, SIGNAL(PropertiesRefreshed()), this,
 			SLOT(AddPreviewButton()));
 	}