filechooser: remove the simple search engine
We should remove the ftw-based search engine and just rely on system indexers for recursive search.
I have the suspicion that many complaints about search in the file chooser come about because tracker is not installed and people end up with this simple engine that is not up to par. It is not used on GTK developer machines, so it is untested, and having it in the mix just makes the experience depend on the environment and adds uncertainty to bug reports.