Remove jira_connect_full_namespace_sync feature flag

What does this MR do?

It removes the jira_connect_full_namespace_sync feature flag. The feature flag was added to be able to turn the feature off in case we see any performance issues. You can read more about it in #281912 (closed). So far I couldn't see any performance issues so we are good to remove the feature flag.

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