Remove vscode_web_ide feature flag

Issue: [Feature flag] Cleanup vscode_web_ide (#526383 - closed)

What does this MR do and why?

Removes vscode_web_ide feature flag and all its references across the codebase. This is a step in the direction of removing the Legacy Web IDE from the GitLab application.

Feature specs removal

We removed several feature specs that targeted the legacy Web IDE. In some cases, there are E2E tests that cover the same features as the removed feature specs. In other cases, the feature covered by the feature spec was never ported to the VSCode-based Web IDE.

More information

For more information about the removal of the legacy Web IDE, see #513938 (closed). We will remove the frontend code in a follow-up Merge Request.

References

Screenshots or screen recordings

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

How to set up and validate locally

This feature flag has been enabled by default since %15.11. This is a cleanup merge request albeit a complex one due to all the technical debt accumulated over the years.

MR acceptance checklist

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