Clean up BE for SecurityConfigurationController

After !76866 (merged) and #333113 (closed) done, we can unify Security::ConfigurationController#show for CE and EE.

I can't be done before because EE version and CE version had different responses for html format (they rendered different views).

Now, backend code can be refactored and we can remove EE #show action.

The unify_security_configuration flag was never enabled and frontend went with a pass of not using json action. More information in !108848 (merged)

We can remove feature flag and clean up the code now.

Implementation plan

1. Delete #show action in EE controller module. Note about permissions: Now security page should be visible for developers of the project without check of the license tier, as SAST and Secret detection features are available in Core.
2. Remove related EE tests for ee#show action.
3. Remove usage of unify_security_configuration fearure flag.