[std::str] Remove the now unused not_utf8 condition.

src/libstd/str.rs

@@ -117,14 +117,6 @@ use default::Default;
 use send_str::{SendStr, SendStrOwned};
 use unstable::raw::Repr;
 
-/*
-Section: Conditions
-*/
-
-condition! {
-    pub not_utf8: (~str) -> ~str;
-}
-
 /*
 Section: Creating a string
 */