Remove linked failure from the runtime

The reasons for doing this are:

* The model on which linked failure is based is inherently complex
* The implementation is also very complex, and there are few remaining who
  fully understand the implementation
* There are existing race conditions in the core context switching function of
  the scheduler, and possibly others.
* It's unclear whether this model of linked failure maps well to a 1:1 threading
  model

Linked failure is often a desired aspect of tasks, but we would like to take a
much more conservative approach in re-implementing linked failure if at all.

Closes #8674
Closes #8318
Closes #8863

src/libstd/path/posix.rs

@@ -565,7 +565,6 @@ mod tests {
             ($name:expr => $code:block) => (
                 {
                     let mut t = task::task();
-                    t.supervised();
                     t.name($name);
                     let res = do t.try $code;
                     assert!(res.is_err());

src/libstd/path/windows.rs

@@ -1290,7 +1290,6 @@ mod tests {
             ($name:expr => $code:block) => (
                 {
                     let mut t = task::task();
-                    t.supervised();
                     t.name($name);
                     let res = do t.try $code;
                     assert!(res.is_err());