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`

docs/contrib/build.md

@@ -17,7 +17,6 @@ You'll need these tools:
 
 You'll need these libraries:
 
--   [GnuTLS](https://www.gnutls.org/)
 -   libuuid (unless on Darwin/BSD)
 
 Specifically the development versions, `uuid-dev` on Debian, for example.

docs/contrib/development.md

@@ -6,7 +6,6 @@ title: How to Build Taskwarrior
  * CMake 3.0 or later
  * gcc 7.0 or later, clang 6.0 or later, or a compiler with full C++17 support
  * libuuid (if not on macOS)
- * gnutls (optional)
  * python 3 (optional, for running the test suite)
 
 ## Obtain and build code:

docs/contrib/first_time.md

@@ -30,13 +30,11 @@ That's just a term that means you need certain tools installed before proceeding
 Here are the tools that Taskwarrior needs:
 
 -   Compiler: GCC 4.7 or newer, or Clang 3.4 or newer.
--   Libraries: GnuTLS, and libuuid
 -   Tools: Git, CMake, make, Python
 
 The procedure for installing this software is OS-dependent, but here are the commands you would use on Debian:
 
     $ sudo apt-get install gcc
-    $ sudo apt-get install libgnutls28-dev
     $ sudo apt-get install uuid-dev
     $ sudo apt-get install git
     $ sudo apt-get install cmake