Make devfile and editor immutable

What does this MR do and why?

Implements Make devfile property immutable (#392759 - closed), to be merged into Remote Development feature behind a feature flag (!105783 - merged)

Also removes hook and worker for devfile in repo.

See https://gitlab.com/gitlab-org/remote-development/gitlab-remote-development-docs for more context.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.