jenkins-docker: Drop obsolete document

Now that we have largely moved away from Jenkins to GitLab CI/CD the
document describing the benefits of using Docker with Jenkins no longer
add much value.

The switch to Docker has proved immensely useful and has been a
fundamental piece for our ability to move to GitLab CI/CD, and in 2020
Docker is widespread enough in the industry that there's no longer a
real need for Apertis to rehash the benefits it brings.

Let's drop the document now that it has been fully implemented and
that we moved the CI/CD architecture even further, making the
document obsolete.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>