[hitbtc] Remove buggy @Produces

Fix #549.
If you look at ResCU [RestInvocationHandler](https://github.com/mmazi/rescu/blob/develop/src/main/java/si/mazi/rescu/RestInvocationHandler.java) lines 77-81 `MediaType.APPLICATION_FORM_URLENCODED` is not valid parameter for `@Produces`.

Works without it.