Remove gitlab-docs/gitlab-docs-lint images from this repository

gitlab-org/gitlab-docs has their own image manifests and registry.

cf. gitlab-docs!662 (merged), gitlab-docs!656 (merged)

While registry.gitlab.com/gitlab-org/gitlab-build-images:gitlab-docs image has been automatically built with Ruby 2.6 for more than 1 year, the project gitlab-org/gitlab-docs is using Ruby 2.7.

Signed-off-by: Takuya Noguchi takninnovationresearch@gmail.com

Closes #47 (closed)