8-17-stable Revert all yarn changes except for yarn.lock

Merged username-removed-636429 requested to merge 8-17-stable-yarn-lock into 8-17-stable

We resolved to make Yarn an optional dependency within 8.17.x so adding a yarn.lock file is all that is necessary. The GitLab omnibus build process will use this internally to ensure the packages we release have deterministic dependencies, but for all users installing from source npm install will still work just fine.

This commit reverts the changes made in gitlab-ce!9591 and simply leaves the yarn.lock file in place.