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/diag.t

@@ -40,9 +40,6 @@ class TestDiagnostics(TestCase):
     def setUp(self):
         self.t = Task()
         self.t.config("editor",            "edlin")
-        self.t.config("taskd.ca",          "/tmp/ca")
-        self.t.config("taskd.trust",       "strict")
-        self.t.config("taskd.credentials", "us/me/xxx")
 
     @unittest.skipIf(
         getattr(platform, 'dist', None) == None or 'xenial' == platform.dist()[-1],
@@ -54,9 +51,8 @@ class TestDiagnostics(TestCase):
         code, out, err = self.t.diag()
         self.tap(out)
         self.assertRegex(out, "Compliance:\s+C\+\+17")
-        self.assertRegex(out, "libgnutls:\s+\d+\.\d+\.\d+")
         self.assertIn("edlin", out)
-        self.assertIn("strict", out)
+        self.assertIn("Locking", out)
 
     def test_64bit_time_t(self):
         """Test that time_t has size of 64 bits"""