Remove unused memoized caches from Context (ruby#1644)

## Summary
- Remove `class_attributes` and `instance_attributes` methods (zero
callers)
- Remove obsolete `instance_method_list` method (deprecated wrapper)
- Remove memoization from `class_method_list` and `instance_methods` —
these are only called once per class/module during generation, so
caching adds no benefit

Author: st0012

lib/rdoc/code_object/context.rb

@@ -640,18 +640,11 @@ def child_name(name)
     end
   end
 
-  ##
-  # Class attributes
-
-  def class_attributes
-    @class_attributes ||= attributes.select { |a| a.singleton }
-  end
-
   ##
   # Class methods
 
   def class_method_list
-    @class_method_list ||= method_list.select { |a| a.singleton }
+    method_list.select { |a| a.singleton }
   end
 
   ##
@@ -951,28 +944,11 @@ def http_url
     File.join(*path.compact) + '.html'
   end
 
-  ##
-  # Instance attributes
-
-  def instance_attributes
-    @instance_attributes ||= attributes.reject { |a| a.singleton }
-  end
-
   ##
   # Instance methods
 
   def instance_methods
-    @instance_methods ||= method_list.reject { |a| a.singleton }
-  end
-
-  ##
-  # Instance methods
-  #--
-  # TODO remove this later
-
-  def instance_method_list
-    warn '#instance_method_list is obsoleted, please use #instance_methods'
-    @instance_methods ||= method_list.reject { |a| a.singleton }
+    method_list.reject { |a| a.singleton }
   end
 
   ##