doc: remove IRC node-dev link from README

The IRC link is mentioned in CONTRIBUTING.md which is linked immediately
above. We should encourage people to read that doc before hoping on the
IRC channel.

PR-URL: https://github.com/nodejs/node/pull/17106
Ref: https://github.com/nodejs/node/pull/16533#discussion_r147292940


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>