Blender Git Loki

Cleanup: skip reading UI datablocks entirely for undo

Other types of datablocks pointing to UI datablocks is unsupported, so
there is no need to store them in fd->libmap. With the experimental undo
speedup enabled preserving such pointers was done. But it didn't work in
2.82 and such pointers are easily lost in cases other than undo.

Differential Revision: https://developer.blender.org/D7329