Remove unused postResponse function from proxy example

In 0d63bafe14f5bfe760bc27e457aca845bce69b0b the `postResponse` function was moved to `examples/proxy.js` but not set as the `postResponse` handler for the proxy.

An alternative to removing the function would be to actually make use of it (see 728f041c8bdf774477a59c4db2b968e1feb78e37).  Since it is listed as deprecated, I thought it would make more sense to remove it from the example.