Revert some warning removals related to Ruby 2.0

These warnings were actually a bug in Ruby 2.0, the accessors should not
be raising such warnings, they are only meant for ivars.

-

Revert "fix warnings in Ruby 2.0"

This reverts commit 26702a6d.

-

Revert "Merge pull request #8282 from arunagw/warning_removed_for_ruby2"

This reverts commit f63d6544, reversing
changes made to 3a890681.

activesupport/lib/active_support/core_ext/date/calculations.rb

@@ -8,8 +8,6 @@
 class Date
   include DateAndTime::Calculations
 
-  @beginning_of_week_default = nil
-
   class << self
     attr_accessor :beginning_of_week_default
 

activesupport/lib/active_support/core_ext/time/zones.rb

 require 'active_support/time_with_zone'
 
 class Time
-  @zone_default = nil
-
   class << self
     attr_accessor :zone_default
 

railties/lib/rails/engine.rb

@@ -407,8 +407,6 @@ def find(path)
       end
     end
 
-    self.isolated = false
-
     delegate :middleware, :root, :paths, to: :config
     delegate :engine_name, :isolated?, to: :class