Remove `realtime_approvals` feature flag

What does this MR do and why?

For #399539 (closed)

Removes the realtime_approvals feature flag that was first released as default_enabled: true in 16.0.0-ee.

Screenshots or screen recordings

This feature flag is already defaulted on, so there's no distinction between the current status and it being completely removed.

How to set up and validate locally

See above re: it's already on by default.

MR acceptance checklist

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