Remove `ci_job_heartbeats_runner` feature flag

What does this MR do?

Related to: #251080 (closed)

Removes already enabled feature flag.

Screenshots

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