[] and read_attribute are not aliases [ci skip]

The `#[]` method *used to be* an alias of `#read_attribute`, but since Rails 4
(10f6f90d9d), it will raise an exception for
missing attributes. Saying that it is an alias is confusing.

activerecord/lib/active_record/attribute_methods.rb

@@ -334,8 +334,6 @@ module ActiveRecord
     #
     # Note: +:id+ is always present.
     #
-    # Alias for the #read_attribute method.
-    #
     #   class Person < ActiveRecord::Base
     #     belongs_to :organization
     #   end