Unable to unselect a key value on mapping field

Here's a quick and dirty patch for this.

I agree this is a nuisance, but the patch supplied doesn't work -- it sets the radio to the first entry in the table (which has a value of 0) instead of unsetting the key.

See:

It's a step in the right direction, though. I'd prefer to see some kind of toggle for the key header "Uses Key" or "No Key" -- something like that, that would allow the validation to work as desired.

I took a stab at this, but got frustrated by how the key works. Should be easier once https://www.drupal.org/node/2565911 is accepted.

OK - annoying to have to write javascript for this, but otherwise this is minimally invasive and solves the problem without waiting for the other, bigger issue.