Remove ignore'd test for old ICE

This test originated from an ICE in Rust 0.4:
373504a7ac (diff-214656000770dff71c8165d051c3f027)

it was ignored one day later:
1fa2b9980f (diff-214656000770dff71c8165d051c3f027)

and then never enabled again. I think we're safe without it.

src/test/run-pass/issue-4120.rs

@@ -1,21 +0,0 @@
-// Copyright 2013-2014 The Rust Project Developers. See the COPYRIGHT
-// file at the top-level directory of this distribution and at
-// http://rust-lang.org/COPYRIGHT.
-//
-// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
-// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
-// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
-// option. This file may not be copied, modified, or distributed
-// except according to those terms.
-
-// ignore-test
-
-// ignored due to Valgrind complaining about TLS loss.
-
-pub fn main()
-{
-    unsafe {
-        libc::exit(0);
-    }
-    println!("ack");
-}