Remove misleading tport example from configs

As an example of using mod_sofia's gateway parameter `contact-params`
we'd used the value `tport=tcp`.  Looking around, it's clear this has
misled people into believing you can specify `tport=tcp` to make the
gateway use TCP or `tport=tls` to make the gateway use TLS.  This does
not work.

The actual contact parameter is named `transport` rather than `tport`,
and you shouldn't use `transport` in `contact-params` because we
automatically add a `transport` to the Contact: based on the value of
`register-transport` (even if the gateway is set to not register).

It's clear why this would be confusing, so we'll just remove this as
an example.

src/mod/endpoints/mod_sofia/conf/sofia.conf.xml

@@ -43,7 +43,7 @@
                calls via this gateway -->
           <param name="caller-id-in-from" value="false"/>
           <!-- extra sip params to send in the contact -->
-          <param name="contact-params" value="tport=tcp"/>
+          <param name="contact-params" value=""/>
           <!-- put the extension in the contact -->
           <param name="extension-in-contact" value="true"/>
           <!-- send an options ping every x seconds, failure will unregister