Fix warnings in stdtest and extratest. Maybe somebody will care.

src/libextra/sort.rs

@@ -1020,8 +1020,6 @@ mod big_tests {
 
     use sort::*;
 
-    use std::cast::unsafe_copy;
-    use std::local_data;
     use std::rand::RngUtil;
     use std::rand;
     use std::uint;

src/libstd/unstable/sync.rs

@@ -357,7 +357,6 @@ pub fn unwrap(self) -> T {
 
 #[cfg(test)]
 mod tests {
-    use super::*;
     use cell::Cell;
     use comm;
     use option::*;