`xmlCleanupCharEncodingHandlers` is deprecated

[nwellnhof]

The function xmlCleanupCharEncodingHandlers is deprecated and will be removed in future libxml2 versions. There's no need to ever call this function. In the best case, it will result in the default char encoding handlers being removed and readded later. In the worst case, it could result in encoding handlers not being found.

This function is called from Source/Common/Policy.cpp.