drop ActsAsArScope#_virtual_column_hash

- `_virtual_column_hash` was dropped: 22153f4 #5982
- Not referenced in our code

Author: kbrock

lib/acts_as_ar_scope.rb

@@ -9,7 +9,7 @@ class << self
 
     delegate :klass, :to => :aar_scope, :prefix => true
     delegate :table_name, :reflections, :to => :aar_scope_klass
-    delegate :_virtual_columns_hash, :virtual_reflections, :to => :aar_scope_klass
+    delegate :virtual_reflections, :to => :aar_scope_klass
   end
 
   def self.aar_scope