Revert "Add custom entrypoint for the build container for Kubernetes executor" changes

What does this MR do?

In !4394 (merged) and !4452 (merged) we improve the support of image entrypoint for the build container for the executorkubernetes

This improvement, even though it was fixing an existing bug #30713 (closed), broke existing setups.

In this MR, we are reverting those changes which will then be reintroduce in %17.0 as breaking change

The MR that will then be re reverted in %17.0 are the following:

• Add custom entrypoint for the build container f... (!4394 - merged) • Baptiste Lalanne • 16.6
• Sanitize image entrypoint to remove empty string (!4452 - merged) • Romuald Atchadé • 16.6

Why was this MR needed?

To revert to an old behaviour and revert a breaking change.

What's the best way to test this MR?

Run a job with the executorkubernetes using the gitlab-ci below. The job should pass

test:
  image:
    name: ubuntu:latest
    entrypoint: ["/bin/sh"]
  script:
    - echo "hi"

Job

What are the relevant issue numbers?

close #37205 (closed) gitlab-org/charts/gitlab-runner#515 (closed) #37233 (closed)