string.c: no terminator

* string.c (rb_str_{,l,r}strip_bang): rb_str_subseq() will not
  NUL-terminate the result string, in the future, so it will not
  be needed in other cases.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48280 b2dd03c8-39d4-4d8f-98ff-823fe69b080e