This reverts commit 4a5456b152.
This commit was here to bypass a bug of dlfp rss Atom.
The bug is fixed and the commit can be reverted.
Reference : http://linuxfr.org/suivi/855
It is unused, and no backend implemented it — so it was wrong to have it
in the function signature.
It also does not seem very helpful, we have get_threads() if we want to
do something advanced with treads.
fixes#825