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/scripts/test_osx.sh

@@ -1,7 +1,6 @@
 set -ex
 
 export LDFLAGS="-framework Foundation -framework Security"
-brew install gnutls
 brew install cmake
 brew install libfaketime
 git clean -dfx