Remove leftover implicit_readonly documentation [ci skip]

Records loaded from join queries are no longer automatically marked as
read-only since aff928ba.

activerecord/lib/active_record/core.rb

@@ -507,8 +507,7 @@ def blank? # :nodoc:
       false
     end
 
-    # Returns +true+ if the record is read only. Records loaded through joins with piggy-back
-    # attributes will be marked as read only since they cannot be saved.
+    # Returns +true+ if the record is read only.
     def readonly?
       @readonly
     end