* Rip out half-implemented m.login.saml2 support
This was implemented in an odd way that left most of the work to the client, in
a way that I really didn't understand. It's going to be a pain to maintain, so
let's start by ripping it out.
* drop undocumented dependency on dateutil
It turns out we were relying on dateutil being pulled in transitively by
pysaml2. There's no need for that bloat.
