Incorrect and unused Is() function

[Hades32]

While looking through the code I noticed this:

backoff/retry.go

Lines 99 to 102 in a78d380

	func (e *PermanentError) Is(target error) bool {
	_, ok := target.(*PermanentError)
	return ok
	}

Go doesn't support (yet) the Is(err) function for use in errors.Is(), and if it did then this implementation is fast but wrong for all wrapped errors.

Can we remove this? I think it's confusing and might break in the future

[seh]

@Hades32, can you clarify the "wrong for all wrapped errors" part? The current implementation of the errors.Is function will progress beyond an error's Is method that doesn't match the target error and inspect the wrapped errors revealed by an error's Unwrap method.