Remove live preview feature from GitLab

What does this MR do and why?

Remove the Live Preview feature from the Web IDE and the ability to enable it at an Admin level.

We will need to follow the deprecation guidelines to follow the code removal with a separate MR for the announcement / documentation removal.

Screenshots or screen recordings

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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.

Related to #374952 (closed)