thread.c: remove unnecessary sentence

* thread.c (thread_start): initialize method is not concerned with
  Thread.start and Thread.fork.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39485 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

thread.c

@@ -668,9 +668,6 @@ thread_s_new(int argc, VALUE *argv, VALUE klass)
  *  Basically the same as ::new. However, if class Thread is subclassed, then
  *  calling +start+ in that subclass will not invoke the subclass's
  *  +initialize+ method.
- *
- *  If you're going to subclass Thread, be sure to call super in your
- *  +initialize+ method, otherwise a ThreadError will be raised.
  */
 
 static VALUE