Remove ThrowIfCancellationRequested from VisitAsync

[Shane32]

As VisitAsync is a very 'hot' path with very little execution time, context.CancellationToken.ThrowIfCancellationRequested(); has been removed for faster execution.

See https://devblogs.microsoft.com/premier-developer/recommended-patterns-for-cancellationtoken/

Consider not checking the token at all if your work is very quick, or you propagate it to the methods you call.

[codecov-commenter]

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