KeysController: return correct number of unsigned prekeys

When GET /v2/keys was orignally added in b263f47, prekeys were stored in
Postgres, with a user's unsigned and signed keys together in the same table.
Therefore GET /v2/keys subtracted one from the count returned by storage.

In d4d9403, we changed to a different storage schema, with unsigned prekeys in
one DynamoDB table and unsigned prekeys in the accounts Dynamo table.
Unfortunately, GET /v2/keys was not changed to stop subtracting one from the
count of prekeys in the keys table at the same time. This commit fixes that.

service/src/main/java/org/whispersystems/textsecuregcm/controllers/KeysController.java

@@ -79,10 +79,6 @@ public class KeysController {
 
     int count = keys.getCount(getIdentifier(auth.getAccount(), identityType), auth.getAuthenticatedDevice().getId());
 
-    if (count > 0) {
-      count = count - 1;
-    }
-
     return new PreKeyCount(count);
   }