Remove 'gitlab' review-app pipeline code

What does this MR do and why?

Remove all code related to gitlab cloud native deployment review-app as it is being removed entirely.

References

Deprecate/Remove gitlab cloud native deployment... (&19358)

Closes Remove review-apps scripts and CI configuration... (#578663 - closed)
Closes Remove review-apps documentation from gitlab-or... (#578665 - closed)

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