types.attrTag: Remove substSubmodules

I haven't managed to trigger the error, and it turns out that this
method is optional.
Specifically, getSubmodules is unimplemented (and unimplementable),
the tests pass, and we seem to have good location info.

lib/types.nix

@@ -662,15 +662,6 @@ rec {
                 };
             })
             tags;
-        substSubModules = m:
-          attrTag
-            (mapAttrs
-              (n: opt:
-                opt // {
-                  type = (opt.type or types.unspecified).substSubModules m;
-                }
-              )
-              tags);
         check = v: isAttrs v && length (attrNames v) == 1 && tags?${head (attrNames v)};
         merge = loc: defs:
           let