Remove :dag_pipeline_tab flag

What does this MR do?

The :dag_pipeline_tab has been on by default since %13.2, so I think we can remove it now.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.