Remove redundant `parent` method from RESTRICTED_CLASS_METHODS

Previously, `parent` was added as one of the RESTRICTED_CLASS_METHODS as
part of a commit (94b7328b08) in 2014 that stopped Rails `enum`s from being
able to redefine important class methods.

At the time, Rails monkey-patched `Module` with a `parent` class method
that returned a module's containing module if it was nested.

However, in October 2020 (167b4153ca) in Rails 6.1, this method was
deprecated in favour of a renamed method `module_parent`. As such, the
`parent` method doesn't need to be a restricted class method any more.

activerecord/lib/active_record/attribute_methods.rb

@@ -20,7 +20,7 @@ module ActiveRecord
       include Serialization
     end
 
-    RESTRICTED_CLASS_METHODS = %w(private public protected allocate new name parent superclass)
+    RESTRICTED_CLASS_METHODS = %w(private public protected allocate new name superclass)
 
     class GeneratedAttributeMethods < Module # :nodoc:
       LOCK = Monitor.new

activerecord/test/cases/enum_test.rb

@@ -535,7 +535,7 @@ class EnumTest < ActiveRecord::TestCase
       :save,     # generates #save!, which conflicts with an AR method
       :proposed, # same value as an existing enum
       :public, :private, :protected, # some important methods on Module and Class
-      :name, :parent, :superclass,
+      :name, :superclass,
       :id        # conflicts with AR querying
     ]
 

activerecord/test/cases/scoping/named_scoping_test.rb

@@ -363,7 +363,6 @@ class NamedScopingTest < ActiveRecord::TestCase
       :protected,
       :private,
       :name,
-      :parent,
       :superclass
     ]