Remove re-exports of std::io::stdio::{print, println} in the prelude.

The `print!` and `println!` macros are now the preferred method of printing, and so there is no reason to export the `stdio` functions in the prelude. The functions have also been replaced by their macro counterparts in the tutorial and other documentation so that newcomers don't get confused about what they should be using.

src/libsyntax/ast_util.rs

@@ -1127,7 +1127,7 @@ mod test {
         // - two renames of the same var.. can only happen if you use
         // local-expand to prevent the inner binding from being renamed
         // during the rename-pass caused by the first:
-        println("about to run bad test");
+        println!("about to run bad test");
         { let sc = unfold_test_sc(~[R(id(a,EMPTY_CTXT),50),
                                     R(id(a,EMPTY_CTXT),51)],
                                   EMPTY_CTXT,&mut t);