Remove junit report feature flag

What does this MR do?

Remove the :junit_pipeline_view feature flag which will enable JUnit Test Reports by default for self hosted customers. This flag is already enabled for the GitLab.com instance.

Related to #216478

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