Get rid of the rust_task_thread::blocked_tasks list

[brson]

This list is only used to locate all tasks when the entire runtime fails. If we implemented #1189 then it wouldn't need to exist at all

[bblum]

I'm not sure it's feasible to get rid of this given the new linked failure mechanism. Even with a message-based tree, spawn_unlinked gives us the possibility of "losing track" of tasks, if we hope to get all of them inductively through failure propagation.

I'd suggest relying on pipes' behaviour of closing-in-destructors, so when failure propagates tasks wouldn't need to get explicitly punted awake, but as long as there's the possibility of deadlocking pipe reads, that can't work either.

In short, not sure it makes sense to do this.

[bblum]

maybe i'll try getting rid of it temporarily and measuring the speed, just to see if this is worth thinking about

[bblum]

I had another look and in the common case it's never even touched except under the same lock as running_tasks. So it can only be as much slowdown as a linked-list manipulation.

Briefly tested removing it (didn't try very hard), saw no difference with vs without. For its functionality, it should stay.