Refactors removing cancellation tokens avoiding mutation of array during iterating it.
Refactors removing cancellation tokens avoiding mutation of array during iterating it.

While refactored code is correct, mutation of collection while iterating it always raises a warning.
Proposed change is more CPU effective as removal of multiple array elements in one call avoids
multiple buffer copying and possible array storage deallocation.
Price for that effectives - need for storage of indices in NSMutableIndexSet.

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