Remove health_status from epics

What does this MR do?

It removes the health_status related code from `epics. We introduced the column in !24202 (diffs) , but for epics we only need to count statuses of child issues as described in #199184 (closed)

Follow-up MR will remove the health_status column from the db.

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.