Add api endpoint for deleting deprecated signaling key

src/main/java/org/whispersystems/textsecuregcm/controllers/AccountController.java

@@ -356,6 +356,14 @@ public class AccountController {
     accounts.update(account);
   }
 
+  @Timed
+  @DELETE
+  @Path("/signaling_key")
+  public void removeSignalingKey(@Auth Account account) {
+    account.getAuthenticatedDevice().get().setSignalingKey(null);
+    accounts.update(account);
+  }
+
   @Timed
   @PUT
   @Path("/attributes/")