Revert "Pass a non-infinite deadline to grpc_completion_queue_next() to prevent queues from blocking indefinitely in poll()"

Pass a non-infinite deadline to grpc_completion_queue_next() to prevent queues from blocking indefinitely in poll().

Because the category makes the protocol apply to all `NSMutableDictionaries`, it can't assume keys are `NSString`. Because these types are covariant, the change is not breaking.

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TODO:
- Documentation
- Make public
- Check I’ve implemented all NSMutDict required methods