configure: remove --with-tls and --disable-dht

Ring no longer builds without GnuTLS and/or without openDHT, nor
are we planning to support building Ring without them, so these
should not exist as configuration options.

The related preprocessor conditionals have also been removed.

Change-Id: I8416ecc48f13a3fb77d9eb7d349b81f226e863f1
Tuleap: #739