Let removeIdentityKey delete the entire record

Previously we would only clear the publicKey since that was the only attribute,
but now we should delete the entire record. This method is currently only called
from tests.

// FREEBIE

js/signal_protocol_store.js

@@ -562,7 +562,7 @@
             return new Promise(function(resolve, reject) {
                 var identityKey = new IdentityKey({id: number});
                 identityKey.fetch().then(function() {
-                    identityKey.save({publicKey: undefined});
+                    identityKey.destroy();
                 }).fail(function() {
                     reject(new Error("Tried to remove identity for unknown number"));
                 });