Remove editor_sticky_table_headers feature flag

What does this MR do and why?

Fully rolls out the editor_sticky_table_headers feature flag by removing it. Sticky table headers for markdown tables are now always enabled in view mode, preview, and the rich text editor.

The feature has been rolled out internally since two milestones. Once all hiccups were fixed, I set it to 100% for customers, so this feature is now live for more than a week. As there has been no reports, I'd like to fully release this feature now and clean up the code again

References

• Rollout issue: #593078 (closed)
• Feature issue: #585265 (closed)
• Introduced by: !226028 (merged)

Screenshots or screen recordings

No visual change — this removes a feature flag that is already fully rolled out. The behavior after this change matches the previously flag-enabled behavior.

How to set up and validate locally

1. Open any rendered markdown table (issue/MR description or comment, wiki page, or the rich text editor).
2. Scroll the page and confirm the table header stays sticky within its scrollable container.