Remove call to BM_elem_attrs_copy in face interpolation function.
This was copying custom-data which cleared the faces CD_BM_ELEM_PYPTR,
making the Python objects behave as if it was removed.
While the issue could also be fixed by skipping the PYPTR layer,
copying attributes such as the material & normal in a face-data
interpolation function isn't correct. In cases where this is needed,
it's better the caller handles copying attributes.
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