* string.c: remvoe STR_ASSOC related code.
  By r44804, string objects can not have STR_ASSOC flag.
* internal.h: ditto.
* ext/objspace/objspace_dump.c (dump_object): ditto.



git-svn-id: svn+ssh://svn.ruby-lang.org/ruby/trunk@44805 b2dd03c8-39d4-4d8f-98ff-823fe69b080e