DescendantsTracker: fix a method redefinition warning

Author: byroot

activesupport/lib/active_support/descendants_tracker.rb

@@ -95,11 +95,6 @@ def reject!(classes) # :nodoc:
       end
     end
 
-    def descendants
-      subclasses = self.subclasses
-      subclasses.concat(subclasses.flat_map(&:descendants))
-    end
-
     if RubyFeatures::CLASS_SUBCLASSES
       class << self
         def subclasses(klass)
@@ -110,6 +105,11 @@ def descendants(klass)
           klass.descendants
         end
       end
+
+      def descendants
+        subclasses = self.subclasses
+        subclasses.concat(subclasses.flat_map(&:descendants))
+      end
     else
       # DescendantsArray is an array that contains weak references to classes.
       # Note: DescendantsArray is redundant with WeakSet, however WeakSet when used