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.

PR-URL: https://github.com/nodejs/node/pull/13231


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>