lib/modules: Remove _module.checks.*.triggerPath as it's not necessary

Previously this option was thought to be necessary to avoid infinite
recursion, but it actually isn't, since the check evaluation isn't fed
back into the module fixed-point.

nixos/modules/misc/assertions.nix

@@ -29,12 +29,6 @@ with lib;
       '';
     };
 
-    _module.checks = mkOption {
-      type = types.attrsOf (types.submodule {
-        triggerPath = mkDefault [ "system" "build" "toplevel" ];
-      });
-    };
-
   };
 
   config._module.checks = lib.listToAttrs (lib.imap1 (n: value: