Remove RADIUS support.

Remove RADIUS support.

Our RADIUS implementation supported only the deprecated RADIUS/UDP
variant, without the recommended Message-Authenticator attribute to
mitigate against the Blast-RADIUS vulnerability.  By now, popular RADIUS
servers are expected to generate loud warnings or reject our
authentication attempts outright.

Since there have been no user reports about this, it seems unlikely that
there are users.

Reviewed-by: Álvaro Herrera <alvherre@kurilemu.de>
Reviewed-by: Aleksander Alekseev <aleksander@tigerdata.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Jacob Champion <jacob.champion@enterprisedb.com>
Reviewed-by: Michael Banck <mbanck@gmx.net>
Discussion: https://postgr.es/m/CA%2BhUKG%2BSH309V8KECU5%3DxuLP9Dks0v9f9UVS2W74fPAE5O21dg%40mail.gmail.com