Draft: Revert "Add assimp_interface to the list of exported targets"

This reverts commits MR !144 (merged) since it introduced build errors in downstream projects.

@reister @pohl If you want, you can merge this MR to revert !144 (merged) and then try to re-apply the changes in a non-breaking way, or you can directly try to fix it (and close this MR unmerged later)