Use 204 instead of 202 for delete of keypairs v3

The delete of keypairs will access db directly. It isn't async call.
So correct it to return 204.

Closes-bug: 1252169
DocImpact

Change-Id: Ie36c16e1773fa7f61b22bd6735342c7308c7ba35
Merge "Use 204 instead of 202 for delete of keypairs v3"