std: Remove msvc/valgrind headers

These aren't really used for anything any more, so there doesn't seem to be much
reason to leave them around in the `rt` directory. There was some limiting of
threads spawned or tests when run under valgrind, but very little is run under
valgrind nowadays so there's also no real use keeping these around.

src/libtest/lib.rs

@@ -41,7 +41,6 @@
 #![feature(fnbox)]
 #![feature(iter_cmp)]
 #![feature(libc)]
-#![feature(rt)]
 #![feature(rustc_private)]
 #![feature(set_stdio)]
 #![feature(staged_api)]
@@ -879,13 +878,7 @@ fn get_concurrency() -> usize {
                 _ => panic!("RUST_TEST_THREADS is `{}`, should be a positive integer.", s)
             }
         }
-        Err(..) => {
-            if std::rt::util::limit_thread_creation_due_to_osx_and_valgrind() {
-                1
-            } else {
-                num_cpus()
-            }
-        }
+        Err(..) => num_cpus(),
     };
 
     #[cfg(windows)]