Remove self monitoring feature flag

What does this MR do?

This removes the feature flag for the self monitor feature that allows the tracking of metrics of a GitLab instance

This depends on the following issues: #42612 (closed) #121719 (closed) #121721 (closed) #198048 (closed)

Original issue: #198511 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides