doc: remove reference to stale CITGM job

Remove reference to job that is out of date, has not
been run for a long time and likely does not work
properly.

Refs: nodejs/citgm#741

Author: mhdawson

COLLABORATOR_GUIDE.md

@@ -194,8 +194,7 @@ for changes that only affect comments or documentation.
 * [`citgm-smoker`](https://ci.nodejs.org/job/citgm-smoker/)
 uses [`CitGM`](https://github.com/nodejs/citgm) to allow you to run
 `npm install && npm test` on a large selection of common modules. This is
-useful to check whether a change will cause breakage in the ecosystem. To test
-Node.js ABI changes you can run [`citgm-abi-smoker`](https://ci.nodejs.org/job/citgm-abi-smoker/).
+useful to check whether a change will cause breakage in the ecosystem.
 
 * [`node-stress-single-test`](https://ci.nodejs.org/job/node-stress-single-test/)
 can run a group of tests over and over on a specific platform. Use it to check