doc: remove Gitter badge from README

Remove the Gitter badge.

Gitter is not supported by us. We use IRC channels on Freenode. Having
the Gitter badge is confusing because we list different resources
later in the doc and never mention Gitter.

Author: Trott

README.md

@@ -4,7 +4,6 @@
   </a>
 </p>
 <p align="center">
-  <a title="Gitter" href="https://gitter.im/nodejs/node?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge"><img src="https://badges.gitter.im/Join%20Chat.svg"></a>
   <a title="CII Best Practices" href="https://bestpractices.coreinfrastructure.org/projects/29"><img src="https://bestpractices.coreinfrastructure.org/projects/29/badge"></a>
 </p>