Remove mastodon_social_ui feature flag

What does this MR do and why?

This MR removes the feature flag mastodon_social_ui.

• See this thread for why the flag was originally introduced.
• Now that Add Generic pre-redirect route for potentially ... (#428446 - closed) is closed, we can remove this flag.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Start GDK and sign in.
2. Visit profile edit page /-/profile and find Mastodon.
3. Enter @me@toot.andr3.net (or any other federated mastodon user ID) and click Update profile settings.
4. Visit user profile page (for example, /root) and find Mastodon icon link.

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 #428163 (closed)