Fix #130113: Copying Object ID never copies its preview.

This was added 7 years ago as 'safe' preservation of previous behavior,
when ID copying was refactored and more control was added over its
behavior.

However, it was never removed since then, even though `NO_PREVIEW` flag
has been part of the `LOCALIZE` copying behavior since many years.

So time to remove this enforced bahevior and use the API as designed. If
this causes new issues, they will have to be fixed in code calling the
ID copy API (most likely by simply adding the `NO_PREVIEW` flag to the
copy options).

source/blender/blenkernel/intern/object.cc

@@ -262,9 +262,7 @@ static void object_copy_data(Main *bmain,
   ob_dst->avs = ob_src->avs;
   ob_dst->mpath = animviz_copy_motionpath(ob_src->mpath);
 
-  /* Do not copy object's preview
-   * (mostly due to the fact renderers create temp copy of objects). */
-  if ((flag & LIB_ID_COPY_NO_PREVIEW) == 0 && false) { /* XXX TODO: temp hack. */
+  if ((flag & LIB_ID_COPY_NO_PREVIEW) == 0) {
     BKE_previewimg_id_copy(&ob_dst->id, &ob_src->id);
   }
   else {