Deprecate / remove `N5Utils.openMipmaps` methods

[This method](https://github.com/saalfeldlab/n5-imglib2/blob/master/src/main/java/org/janelia/saalfeldlab/n5/imglib2/N5Utils.java#L625) and others depend on the presence of specific metadata.  The n5-universe and n5-viewer repos currently handle this functionality in a more metadata-agnostic way.
