deprecate alfalafa in favour of ned

The `alfaalfa` url (http://arecibo.tc.cornell.edu/hiarchive/alfalfa/spectraFITS) point to ned, so I wonder why we keep it as a separate service (other than for historical reasons).

Start by locating the astroquery entry points for the AlfaAlfa service and the Ned service, then compare how each uses the listed AlfaAlfa URL. Confirm the intended deprecation path and identify any service references that would need to change; done means AlfaAlfa is deprecated in favor of Ned without breaking the supported query interface.

Written by the indexing model from the issue text.