Remove the SecurityManagerConfig class

What does this MR do and why?

The Security Manager rollout gate is fully removed, so Gitlab::Security::SecurityManagerConfig has no callers left. Its own definition was the last reference to it anywhere in the repo.

This is the last change in the series. The env var, the spec tag, all 9 call sites and the frontend flag are already on master.

How to set up and validate locally

bundle exec rails runner 'Rails.application.eager_load!; puts defined?(Gitlab::Security::SecurityManagerConfig).inspect'

Eager loads clean and prints nil.

References

#584144 (closed)

Supersedes the remainder of !239507 (closed)

Screenshots or screen recordings

No UI change.

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