Removed Slack link. Focusing on Discord community.

index.html

@@ -61,7 +61,6 @@
 						<li><a href="https://stackoverflow.com/questions/tagged/three.js">questions</a></li>
 						<li><a href="https://discord.gg/56GBJwAnUS">discord</a></li>
 						<li><a href="https://discourse.threejs.org/">forum</a></li>
-						<li><a href="https://join.slack.com/t/threejs/shared_invite/zt-rnuegz5e-FQpc6YboDVW~5idlp7GfDw">slack</a></li>
 						<li><a href="https://twitter.com/threejs">twitter</a></li>
 					</ul>