services.xserver.startGnuPGAgent: remove obsolete NixOS option

GnuPG 2.1.x changed the way the gpg-agent works, and that new approach no
longer requires (or even supports) the "start everything as a child of the
agent" scheme we've implemented in NixOS for older versions.

To configure the gpg-agent for your X session, add the following code to
~/.xsession or some other appropriate place that's sourced at start-up:

    gpg-connect-agent /bye
    GPG_TTY=$(tty)
    export GPG_TTY

If you want to use gpg-agent for SSH, too, also add the settings

    unset SSH_AGENT_PID
    export SSH_AUTH_SOCK="${HOME}/.gnupg/S.gpg-agent.ssh"

and make sure that

    enable-ssh-support

is included in your ~/.gnupg/gpg-agent.conf.

The gpg-agent(1) man page has more details about this subject, i.e. in the
"EXAMPLES" section.

nixos/modules/rename.nix

@@ -111,6 +111,7 @@ with lib;
     (mkRemovedOptionModule [ "services" "openvpn" "enable" ])
     (mkRemovedOptionModule [ "services" "printing" "cupsFilesConf" ])
     (mkRemovedOptionModule [ "services" "printing" "cupsdConf" ])
+    (mkRemovedOptionModule [ "services" "xserver" "startGnuPGAgent" ])
 
   ];
 }