This was never used, so let's get rid of it.

this is never set to anything other than "test", and is a source of unnecessary
boilerplate.

because, frankly, it looked like it was written by an axe-murderer.

This should be a non-functional change, except that where `m.login.dummy` was
previously advertised *before* `m.login.terms`, it will now be advertised
afterwards. AFAICT that should have no effect, and will be more consistent with
the flows that involve passing a 3pid.

By setting the config value directly, we skip the block that adds the slash automatically for us.

So people can still collect consent the old way if they want to.