lib.types.submodule: Remove redundant isSubmodule attr

lib/modules.nix

@@ -505,7 +505,7 @@ rec {
               firstOption = findFirst (m: isOption m.options) "" decls;
               firstNonOption = findFirst (m: !isOption m.options) "" decls;
             in
-              if firstOption.options.type?isSubmodule && firstOption.options.type.isSubmodule
+              if firstOption.options.type.name == "submodule"
               then
                 let opt = fixupOptionType loc (mergeOptionDecls loc (map optionTreeToOption decls));
                 in {

lib/types.nix

@@ -642,11 +642,6 @@ rec {
               else throw "A submoduleWith option is declared multiple times with conflicting shorthandOnlyDefinesConfig values";
           };
         };
-      } // {
-        # Submodule-typed options get special treatment in order to facilitate
-        # certain migrations, such as the addition of freeformTypes onto
-        # existing option trees.
-        isSubmodule = true;
       };
 
     # A value from a set of allowed ones.