rename std::vec_ng -> std::vec

Closes #12771

src/libsyntax/util/parser_testing.rs

@@ -15,7 +15,7 @@ use parse::{new_parser_from_source_str};
 use parse::parser::Parser;
 use parse::token;
 
-use std::vec_ng::Vec;
+use std::vec::Vec;
 
 // map a string to tts, using a made-up filename:
 pub fn string_to_tts(source_str: ~str) -> Vec<ast::TokenTree> {