Remove feature checking for Class#descendants

Ref: https://bugs.ruby-lang.org/issues/14394#note-38

Based on Matz's last comment, it's not so clear whether `Class#descendants`
will come back in the same form or at all. So let's not assume anything.

activesupport/lib/active_support/core_ext/class/subclasses.rb

@@ -3,7 +3,6 @@
 require "active_support/ruby_features"
 
 class Class
-  unless ActiveSupport::RubyFeatures::CLASS_DESCENDANTS
   if ActiveSupport::RubyFeatures::CLASS_SUBCLASSES
     def descendants
       subclasses.concat(subclasses.flat_map(&:descendants))
@@ -28,7 +27,6 @@ class Class
       end
     end
   end
-  end
 
   # Returns an array with the direct children of +self+.
   #

activesupport/lib/active_support/descendants_tracker.rb

@@ -51,7 +51,6 @@ module ActiveSupport
           unless @clear_disabled
             @clear_disabled = true
             remove_method(:subclasses)
-            remove_method(:descendants) if RubyFeatures::CLASS_DESCENDANTS
             @@excluded_descendants = nil
           end
         end
@@ -86,17 +85,9 @@ module ActiveSupport
         subclasses
       end
 
-      if RubyFeatures::CLASS_DESCENDANTS
-        def descendants
-          descendants = super
-          descendants.reject! { |d| @@excluded_descendants[d] }
-          descendants
-        end
-      else
       def descendants
         subclasses.concat(subclasses.flat_map(&:descendants))
       end
-      end
 
       def direct_descendants
         ActiveSupport::Deprecation.warn(<<~MSG)

activesupport/lib/active_support/ruby_features.rb

@@ -3,6 +3,5 @@
 module ActiveSupport
   module RubyFeatures # :nodoc:
     CLASS_SUBCLASSES = Class.method_defined?(:subclasses) # RUBY_VERSION >= "3.1"
-    CLASS_DESCENDANTS = Class.method_defined?(:descendants)
   end
 end