Fix #120861: BMFace.copy_from_face_interp looses reference

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.

source/blender/bmesh/intern/bmesh_interp.c

@@ -150,10 +150,6 @@ void BM_face_interp_from_face_ex(BMesh *bm,
   float *w = BLI_array_alloca(w, f_src->len);
   float co[2];
 
-  if (f_src != f_dst) {
-    BM_elem_attrs_copy(bm, bm, f_src, f_dst);
-  }
-
   /* interpolate */
   l_iter = l_first = BM_FACE_FIRST_LOOP(f_dst);
   do {

source/blender/bmesh/intern/bmesh_interp.h

@@ -88,6 +88,8 @@ void BM_elem_float_data_set(CustomData *cd, void *element, int type, float val);
  * Projects target onto source, and pulls interpolated custom-data from source.
  *
  * \note Only handles loop custom-data. multi-res is handled.
+ * \note Attributes such as selection, material & normals
+ * must be handled with a separate call to #BM_elem_attrs_copy.
  */
 void BM_face_interp_from_face_ex(BMesh *bm,
                                  BMFace *f_dst,