Remove reference to Kaniko docs

What does this MR do?

Removes the reference to Kaniko since there's no known way to use it with the Fargate driver at present. Kaniko's README states states that:

kaniko is meant to be run as an image: gcr.io/kaniko-project/executor. We do not recommend running the kaniko executor binary in another image, as it might not work.

Running kaniko in any Docker image other than the official kaniko image is not supported (ie YMMV). This includes copying the kaniko executables from the official image into another image.

As users currently need to provide a custom image with at least SSH support to run their pipelines on AWS Fargate using the Custom Executor driver, the approach suggested in GitLab/Kaniko docs doesn't fit their needs. Also, I've worked on a PoC in which I tried to build a custom Kaniko image by adding an SSH server and gitlab-runner binary and didn't succeed. 😔

Based on the above reasoning I suggest not including Kaniko in the docs for now.

PS: Maybe instead of removing the references we can let it clear why Kaniko does not work with the driver for now. 🤔