At this point, LLUICtrlFactory::getLayeredXMLNode() is a pretty thin wrapper
around LLDir::findSkinnedFilenames() and LLXMLNode::getLayeredXMLNode().
Until now, LLUICtrlFactory::getLayeredXMLNode() passed (by default)
LLDir::CURRENT_SKIN to LLDir::findSkinnedFilenames(). But that meant that a
caller such as LLTransUtil::parseStrings() that wants almost the same
functionality, but with LLDir::ALL_SKINS instead, had to clone the logic from
LLUICtrlFactory::getLayeredXMLNode(). Allowing its caller to pass the desired
LLDir::ESkinConstraint enum value eliminates the need to clone its logic.
Remove cloned logic from LLTransUtil::parseStrings().