Fix Status Page active status and remove unused StatusPageSetting

What does this MR do?

In !28966 (merged) we've removed the beta feature flag to make Status Page generally available.

Thereafter, in !29655 (merged) we've moved StatusPageSetting to StatusPage::ProjectSetting.

Apparently, we've missed to remove this model and just added StatusPage::ProjectSetting.

Addionally, due to a wrong resolution of a merge conflict we've added back the beta feature flag to the new model StatusPage::ProjectSetting. This made the Status Page setting page behave weirdly: It showed up but users weren't able to activate.

This commits fixes both issues:

• Remove unused StatusPageSetting model
• Remove beta feature flag from StatusPage::ProjectSetting

Although a ~bug, this was never released to self-managed users hence no changelog.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team