prevent memory leak from RepoTreeModel

object ``virtual_repos_catetory_`` is created without being used. 
this patch simply comments this line out to prevent memory leak.
With this line commented out, `virtual_repo_cateory_` would be unintialized, and [this member function](https://github.com/haiwen/seafile-client/blob/v3.1.6/src/ui/repo-tree-model.cpp#L166) would be in trouble.