Remove web_ide_multi_domain feature flag

Issue: [Feature flag] Rollout of `web_ide_multi_domain` (#526529 - closed)

What does this MR do and why?

It removes the web_ide_multi_domain feature flag references and definition. This feature flag controls the enablement of the Web IDE multi-domain architecture on gitlab.com. After removing this feature flag, this architecture will be permanently enabled only in SaaS (gitlab.com).

For how long has this feature flag been enabled globally?

20 days #526529 (comment 2490493456).

References

Screenshots or screen recordings

This Merge Request doesn't introduce user-facing changes.

How to set up and validate locally

This MR doesn't introduce new behaviors to the Web IDE.

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