Cleanup after removal of `jira_connect_oauth` feature flag
What does this MR do and why?
Follow-up from !117826 (merged).
Removes most of the unused code from removal of the jira_connect_oauth
feature flag. Review each commit for a summary of changes.
Screenshots or screen recordings
No changes to UI.
How to set up and validate locally
https://docs.gitlab.com/ee/development/integrations/jira_connect.html#simple-setup
MR acceptance checklist
This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.
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I have evaluated the MR acceptance checklist for this MR.
Related to #355048 (closed)