Remove incorrect comment regarding #initialize_copy

Comment added in #3164 is incorrect after reading http://dev.rubyonrails.org/ticket/7191.

Signed-off-by: José Valim <jose.valim@gmail.com>

activerecord/lib/active_record/base.rb

@@ -1413,14 +1413,6 @@ def initialize(attributes = nil)
       # as it copies the object's attributes only, not its associations. The extent of a "deep" clone is
       # application specific and is therefore left to the application to implement according to its need.
       def initialize_copy(other)
-        # Think the assertion which fails if the after_initialize callback goes at the end of the method is wrong. The
-        # deleted clone method called new which therefore called the after_initialize callback. It then went on to copy
-        # over the attributes. But if it's copying the attributes afterwards then it hasn't finished initializing right?
-        # For example in the test suite the topic model's after_initialize method sets the author_email_address to
-        # test@test.com. I would have thought this would mean that all cloned models would have an author email address
-        # of test@test.com. However the test_clone test method seems to test that this is not the case. As a result the
-        # after_initialize callback has to be run *before* the copying of the attributes rather than afterwards in order
-        # for all tests to pass. This makes no sense to me.
         callback(:after_initialize) if respond_to_without_attributes?(:after_initialize)
         cloned_attributes = other.clone_attributes(:read_attribute_before_type_cast)
         cloned_attributes.delete(self.class.primary_key)