doc: update Collaborators list in README

The Collaborators list in the README has a couple entries of people that
have left the Collaborators team in GitHub. This updates the list in the
README accordingly.

PR-URL: https://github.com/nodejs/node/pull/9846
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

README.md

@@ -217,8 +217,6 @@ more information about the governance of the Node.js project, see
 **Claudio Rodriguez** <cjrodr@yahoo.com>
 * [danbev](https://github.com/danbev) -
 **Daniel Bevenius** <daniel.bevenius@gmail.com>
-* [domenic](https://github.com/domenic) -
-**Domenic Denicola** <d@domenic.me>
 * [eljefedelrodeodeljefe](https://github.com/eljefedelrodeodeljefe) -
 **Robert Jefe Lindstaedt** <robert.lindstaedt@gmail.com>
 * [estliberitas](https://github.com/estliberitas) -
@@ -337,8 +335,6 @@ more information about the governance of the Node.js project, see
 **Yorkie Liu** <yorkiefixer@gmail.com>
 * [yosuke-furukawa](https://github.com/yosuke-furukawa) -
 **Yosuke Furukawa** <yosuke.furukawa@gmail.com>
-* [zkat](https://github.com/zkat) -
-**Kat Marchán** <kzm@sykosomatic.org>
 
 Collaborators (which includes CTC members) follow the
 [COLLABORATOR_GUIDE.md](./COLLABORATOR_GUIDE.md) in maintaining the Node.js