string.c: remove old interfaces

* string.c (rb_str_new[2-5], rb_{tainted,usascii}_str_new2),
  (rb_str_buf_new2): remove old interfaces.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41016 b2dd03c8-39d4-4d8f-98ff-823fe69b080e