Remove switch editing controls feature flag

What does this MR do and why?

It removes the feature flag that hides the switch control to change between Rich text and Source editing modes in the Wiki. This feature flag is enabled by default both on self-managed and gitlab.com. This MR also removes all the references to the previous UI in documentation and QA tests.

Screenshots or screen recordings

Before	After

How to set up and validate locally

You can try editing a Wiki page using both the Content Editor and the Classic Markdown Editor in the Wiki.

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.

Relates to #345398 (closed)