channels: remove WeakRef from Condition

[vtjnash]

Using a WeakRef meant we might not actually bind the result.
If nobody was still holding a reference to put contents into the Condition,
we would simply garbage collect it, and then never need to close it.
Since that does not seem to be the intent,
instead move to just keeping a strong reference
(alternatively, we would have to switch to using stream_wait
with ref-counting, but that seems suboptimal for several reasons.).

fix #31507

[JeffBezanson]

For those following along at home, and to see if I understand this, let me try to summarize. Consider the following task:

@async begin
    c = Channel(0)
    wait(c)
    fire_missiles()
end

Do the missiles ever get fired? No; nobody else has a reference to the channel, so wait will never return and the whole channel-and-task-bundle can just be GC'd. Now change it to:

@async begin
    c = Channel(0)
    bind(c, @async 0)
    try wait(c); catch; end
    fire_missiles()
end

bind says to close c when the @async 0 task exits. That would wake up the wait with an error, firing the missiles. But currently, we don't bother closing the channel if it has been GC'd (via a WeakRef), so whether the missiles fire is unpredictable. This PR changes it so that bind will always close the channel if its argument task exits.

In this case, "fire the missiles" means "run the rest of the test suite" :)