Inline uneccessary frozen string constant

We are only supporting Ruby 2.2 and later in Rails 5, so we do not need
an actual constant here. Additionally, referencing a constant actually
does a hash lookup (because constants are not constant in Ruby >_>).
This will be marginally (likely immeasurable) faster. It is less ugly.

activerecord/lib/active_record/attribute_methods/read.rb

@@ -56,14 +56,12 @@ def #{temp_method}
         end
       end
 
-      ID = 'id'.freeze
-
       # Returns the value of the attribute identified by <tt>attr_name</tt> after
       # it has been typecast (for example, "2004-12-12" in a date column is cast
       # to a date object, like Date.new(2004, 12, 12)).
       def read_attribute(attr_name, &block)
         name = attr_name.to_s
-        name = self.class.primary_key if name == ID
+        name = self.class.primary_key if name == 'id'.freeze
         _read_attribute(name, &block)
       end