UI: Don't clear lastService immediately after setting it

Fixes a bug where when you clicked "Show more..." in the service
settings combobox, the currently selected service would be changed to
the first service in the list.

UI/window-basic-settings-stream.cpp

@@ -185,7 +185,6 @@ void OBSBasicSettings::LoadStream1Settings()
 	else
 		ui->key->setText(key);
 
-	lastService.clear();
 	ServiceChanged();
 
 	UpdateKeyLink();