Remove X/Twitter link from homepage social media section

The PHP project appears to have moved away from X (see
ThePHPF/thephp.foundation#194). Removes the link from the homepage
sidebar while keeping Mastodon and LinkedIn.

Closes #1877

Author: Nelson Castillo

index.php

@@ -181,12 +181,6 @@
       <span class='headline'>Social media</span>
       <div class='body'>
         <ul>
-          <li>
-            <a href="https://twitter.com/official_php" target="_blank" rel="noopener noreferrer">
-              <i class="icon-x-twitter"></i>
-              @official_php
-            </a>
-          </li>
           <li>
             <a href="https://fosstodon.org/@php" target="_blank" rel="noopener noreferrer">
               <i class="icon-mastodon"></i>