Fail in CloneGDIRefData() instead of implementing it incorrectly in wxOSX.

Fail in CloneGDIRefData() instead of implementing it incorrectly in wxOSX.

wxIcon and wxMetaFile implemented CloneGDIRefData() using copy ctors of the
corresponding ref data classes but the copy ctors were either wrong (for
wxIconRefData as using it would result in messing up IconRef reference count)
or had wrong semantics (wxMetafileRefData copy ctor performed shallow copy
only while CloneGDIRefData() supposes a deep copy is done).

Replace the wrong implementations of these functions with assert that will be
triggered if they are ever used (which doesn't seem to be the case so far).

See #12768.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66373 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775