Remove the downweight_closed_orgs debug parameter

I've been testing this with all the examples we've currently got, and it
downweights the organisations correctly for the example searches, but
still returns them at or near the top for searches for the exact name of
the closed organisation.  Therefore, let's just deploy this without the
parameter.
Do we need both this and the `closed_org_boost` in the `boost_filters` method?