Reduce the size of the Docker lint image

The Docker lint image is now almost 1.6Gb. I was wondering why it was this big, and I did a little research.

This MR removes the /usr/lib/go directory which is about 250MB. It's only used to compile Vale and we don't need it afterwards. Tested locally with:

git checkout reduce-size-of-docker-lint
docker build -t docs-lint -f dockerfiles/Dockerfile.gitlab-docs-lint .
cd to_gitlab_repo
docker run -it --rm -v $PWD:/code docs-lint bash
cd /code
./scripts/lint-doc.sh