Removing imports of std::vec_ng::Vec

It's now in the prelude.

src/test/compile-fail/issue-11873.rs

@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-use std::vec::Vec;
-
 fn main() {
     let mut v = vec!(1);
     let f = || v.push(2);

src/test/run-pass/overloaded-autoderef-count.rs

@@ -10,7 +10,6 @@
 
 use std::cell::Cell;
 use std::ops::{Deref, DerefMut};
-use std::vec::Vec;
 
 #[deriving(Eq)]
 struct DerefCounter<T> {

src/test/run-pass/typeck_type_placeholder_1.rs

@@ -14,8 +14,6 @@
 static CONSTEXPR: *int = &'static 413 as *_;
 
 pub fn main() {
-    use std::vec::Vec;
-
     let x: Vec<_> = range(0u, 5).collect();
     assert_eq!(x.as_slice(), &[0u,1,2,3,4]);