Deprecate IDREF-related functions in valid.h

These functions are only needed internally for validation.

xmlGetRefs is inherently unsafe because the ref table isn't updated
if attributes are removed (unlike the ids table).

None of the Ubuntu 20.04 packages depending on libxml2 use any of these
functions (except xmlFreeRefTable in libxslt), so it seems perfectly
safe to deprecate them.

Remove xmlIsRef and xmlRemoveRef from the Python bindings.