Sync against taskchampion-sync-server (#3118)

This removes use of gnutls and the TLS implementation, which is no
longer needed (task synchronization is handled via Taskchampion, which
uses `reqwest`, which handles TLS via other Rust dependencies). This
incidentally removes the following config options:
 * `debug.tls`
 * `taskd.ca`
 * `taskd.certificate`
 * `taskd.ciphers`
 * `taskd.credentials`
 * `taskd.key`
 * `taskd.server`
 * `taskd.trust`

src/feedback.cpp

@@ -207,8 +207,7 @@ void feedback_unblocked (const Task& task)
 ///////////////////////////////////////////////////////////////////////////////
 void feedback_backlog ()
 {
-  if (Context::getContext ().config.get ("taskd.server") != "" &&
-      Context::getContext ().verbose ("sync"))
+  if (Context::getContext ().verbose ("sync"))
   {
     int count = Context::getContext ().tdb2.num_local_changes ();
     if (count)