Text repr: remove "Dimensions without coordinates" section

[benbovy]

The Dimensions without coordinates section of the Dataset and DataArray text reprs was useful in the context of dimension coordinates with a default index. Since the index refactor, however, it may be more confusing than useful and even inaccurate in the case of non-dimension coordinate or multi-coordinate indexes.

I took the bold step of simply removing that section in this PR. I don't know who is really paying attention to it nowadays, though (there's no equivalent in the html reprs). Alternatively we could keep it and try listing the dimensions more accurately.

[shoyer]

I was never a big fan of "Dimensions without coordinates"!

This message does provide a gentle nudge to annotate coordinates on all dimensions, but I'm not sure that's appropriate or necessary. Xarray would definitely be a bit tidier for non-coordinate users if this section was not shown.

Digging back through when we added this, these issues provide the original context:
#1017 (comment)
#1199 (comment)

In particular, the complaint was that it could be hard to notice missing coordinates on DataArray because coordinates can be far below the list of dimensions. So maybe part of this fix could be reversing the order of "Data" and "Coordinates" in the DataArray repr?