Null efficiencies for DS20

[mcolomermolla]

We should remove the eff files for DS20 as the efficiency is zero and makes the prediction for the number of events null.

[JostMigenda]

Note: See SNEWS2/snewpy#295 for earlier discussion.

Agree that an energy-independent efficiency of 0 for all channels is not useful. We can delete those files completely for now (so that e.g. SNEWPY would use the default efficiency of 1.0 instead). Are you working on some more realistic efficiency files to be added in the future?

[mcolomermolla]

Yes, I followed the discussion, but they were never removed.
The new file to be included is the file I sent to you on slack, provided by Sebastian in the same issue, but that I didn't get an answer yet from Andrey on the best way of handling it, as it has not fixed binning. I attach it here.
ds20k_sn_nu_eff.csv