Resolve "[Feature flag] Clean up Jira Connect OAuth"

What does this MR do and why?

As per #355048 (closed) we're removing the jira_connect_oauth feature flag. The feature has been enabled since 7 months ago.

Screenshots or screen recordings

N/A

How to set up and validate locally

N/A

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #355048 (closed)