astroquery.ibe's IbeClass's list_datasets not working as intended

After applying the fix in pull request #1426 I found that the remaining list methods don't work as intended. The origin of this error seems to be that the IRSA ibe service is images only, so that level is trivial and skipped, as you can confirm by browsing the [ibe schema](http://irsa.ipac.caltech.edu/ibe/search/) manually. I'm proposing a fix to this issue (and the rest of the list methods) in a new pull request momentarily, where I render the `list_datasets` function trivial and bypass it elsewhere.

The rationale behind this approach is that I assume that the astroquery API requires the module to have the `list_missions`, `list_datasets`, and `list_tables` functions. If that's so, it seems that the most consistent way to satisfy this requirement is the approach proposed here.

This issue is primarily for discussion of whether this approach is the correct one, and coordinate alternate fixes, if needed.

Browse the IRSA IBE schema at the linked service and inspect astroquery.ibe's IbeClass list methods. Determine whether the API requires list_missions, list_datasets, and list_tables when the service exposes images only, then coordinate the alternative fixes described in the issue. Done means the chosen approach is agreed and the remaining list methods behave consistently with the schema.

Written by the indexing model from the issue text.