Remove multi-line suggestions feature flag

What does this MR do?

That's a straightforward feature flag code removal. Ideally we should add a ~"Pick into 11.10" as soon as we guarantee things are working well in production.

Does this MR meet the acceptance criteria?

Closes #59178 (closed)

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content