Revert "Cancellable forEach"

[benlesh]

Reverts #3977

Currently, there's an issue with what is in there if you unsubscribe from a synchronous observable, synchronously, but after the observable completes:

const subs = new Subscription();

of('test').forEach(x => console.log(x), subs)
  .then(() => console.log('this should not be called'));

subs.unsubscribe();

// Logs
// "test"
// "this should not be called"

Solutions around this aren't too bad, but it adds a lot of bloat, mostly due to the fact we're accepting PromiseConstructorLike as an argument, and we're beholden to use it.

I want to revert this new feature until:

1. We deprecate and remove all Promise Constructor configuration (This is legacy, and users should use a proper polyfill)
2. We deprecate and update the behavior of forEach to call it's next handler on the next microtask, as per the proposals. (We can't do this until v7 because it's a breaking change)

I still want to use Subscriptions as a cancellation here, but it's going to have to wait.

Sorry folks.

[benlesh]

NOTE: We'll still need to redo the fix to forEach to make sure additional emissions aren't still happening after an error is thrown.

[rxjs-bot]

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[coveralls]

Coverage decreased (-0.005%) to 96.947% when pulling f882647 on revert-3977-cancellable-forEach into bb77e25 on master.

[cartant]

Disappointing that the reversion is necessary, but LGTM.