Remove unused 'mem' import in 'os_str' module

Author: nvzqz

src/libstd/ffi/os_str.rs

@@ -10,7 +10,6 @@
 
 use borrow::{Borrow, Cow};
 use fmt;
-use mem;
 use ops;
 use cmp;
 use hash::{Hash, Hasher};