Remove usage of numpy's private functions

[sciunto]

In skimage/util/arraycrop.py, we are using _as_pairs which belongs to numpy and is a private function.

The main issue is that the API may change at any time (and did happen). We must consider adding this code in our library.

Bonus: check if other private functions are used elsewhere (likely not).

[Harry-Kwon]

Hi I've attempted to modify the method to remove to reference to numpy's private functions in this pull request #5081. Are these modifications ok?

I also didn't find any other uses of numpy's private functions in the codebase.

[jni]

Closed by #5081