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`

ChangeLog

@@ -1,3 +1,16 @@
+- [BREAKING CHANGE] sync is now performed against taskchampion-sync-server,
+  instead of taskd. The following config options are no longer supported:
+  - `debug.tls`
+  - `taskd.ca`
+  - `taskd.certificate`
+  - `taskd.ciphers`
+  - `taskd.credentials`
+  - `taskd.key`
+  - `taskd.server`
+  - `taskd.trust`
+
+The Taskwarrior build no longer requires GnuTLS.
+
 ------ current release ---------------------------
 2.6.2 -