title: Unblocking url using webRequest API by removing the listener which has blocked it
body: <p>After getting a clue from this post
  <a href="https://stackoverflow.com/questions/51626117/chrome-extension-to-unblock-a-blocked-url-using-webrequest">Post</a></p>
  <p>I am trying to remove a Listener (which has blocked the URL using webRequest api) to unblock the URL.
  But I am not able to remove it successfully, I am doing something like this...</p>
  <p><strong>To block the URL</strong></p>
  <pre><code>chrome.webRequest.onBeforeRequest.addListener( function blockListener (details) {
                          return {
                              cancel: true
                          };
                      },{ urls: [url], types: [ 'main_frame' ] }, ['blocking'] );
  </code></pre>
  <p><strong>To unblock the URL</strong></p>
  <pre><code>chrome.webRequest.onBeforeRequest.removeListener(blockListener);
  </code></pre>
  <p>What am I doing wrong?</p>