Remove doc-comment default::Default imports

In 8f5b5f94dc, `default::Default` was
added to the prelude, so these imports are no longer necessary.

src/libstd/tuple.rs

@@ -43,8 +43,6 @@
 //! Using traits implemented for tuples:
 //!
 //! ```
-//! use std::default::Default;
-//!
 //! let a = (1, 2);
 //! let b = (3, 4);
 //! assert!(a != b);