Right click in the white space of the model navigator and switch out of timestamp mode. You should now see a folder called 'unused objects', expand that folder and you can delete the lightweight objects there.
The lightweight objects are useful to have around if you have large assemblies, but if like me you have multiple solid bodies in the same file (tool solids) they can get annoying. The system will probably recreate the lightweight solids the next time you save the file unless you have turned this option off (it is probably hiding there somewhere, but I have never bothered to look).