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`

test/run_all

@@ -22,9 +22,6 @@ except ImportError:
     # python 3
     from queue import Queue, Empty
 
-# Look for taskd in $PATH instead of task/src/
-os.environ["TASKD_USE_PATH"] = "1"
-
 TIMEOUT = .2