Remove signald service

The signald project is unmaintained and has long been incompatible with
the official Signal servers.

modules/matrix/mautrix-signal.nix

@@ -43,8 +43,4 @@ lib.mkIf cfg.mautrix-signal.enable {
       };
     } // cfg.mautrix-signal.settings;
   };
-
-  services.signald = {
-    enable = true;
-  };
 }