remove GitLab CI configuration

Stop trying to run a job depending on the java8 tag -- there are no runners for the java8 tag.

Currently the attempts to run the pipeline are just distracting and getting in the way of merging change and testing in predev.

Someday it'd be nice to have an available java8 runner and to run GitLab CI pipeline on this. It'd catch if it were ever so broken that it wouldn't build, and it'd catch it early, before predev. However, we're not giving up very much if we test the build when changes merge to master and either make it to predev or don't. It'll be fine.