Remove skimage specific array expression syntax

[lagru]

Which was somewhat ambiguous and more complicated than necessary. In the recent skimage sprint we agreed that we don't want to use it going forward. Instead let's go with the convention used by scikit-learn.

Release note

For maintainers and optionally contributors, please refer to changelist's README on how to document this PR for the release notes.

Remove array expressions inspired by scikit-image. This impacts forms like
`(M, 3) int array`. Instead, prefer the more expressive convention used by
scikit-learn, for example, `array of dtype int and shape (M, 3)`.