Remove context deadline checking in backOffContext

[swithek]

The current implementation of backOffContext checks the context's deadline and marks it as cancelled even when that deadline is not reached yet. This leads to confusion on the RetryNotifyWithTimer caller's side, because there are no clear indications whether the execution was stopped due to the context reaching its deadline (context.DeadlineExceeded is not returned) or some other error. Even checking whether the context is cancelled after the retry func returns, does not work because the context itself might not have been cancelled (only backOffContext marked it internally as being so):

b := backoff.WithContext(backoff.NewConstantBackOff(time.Second), ctx)
err := backoff.Retry(operation, b)
if err != nil {
      if cerr := ctx.Err(); cerr != nil { // there might be no error returned from the context because backOffContext only 'predicts' its cancellation
            //...
      }
}

[coveralls]

Coverage increased (+0.8%) to 88.987% when pulling aac57c9 on swithek:remove-ctx-deadline into c2975ff on cenkalti:v4.