Blender Git Loki

Cleanup: Remove unused/unneeded code from old Asset Engine design

This code was written for the File Browser together with the Asset Engine
design, that is not part of the Asset Browser/System design anymore. Updated
comments accordingly.

`FileDirEntryRevision` was actually used, but I removed it and moved the used
members to the parent `FileDirEntry`, since there is no concept of revisions
currently.

There should be no functional changes.