Avoid unnecessary records in project factory for jira_integration trait

What does this MR do and why?

Contributes to #378183

Screenshots or screen recordings

I ran the following command, based on the help I asked over discord

FPROF=1 bundle exec rspec spec/models/concerns/mentionable_spec.rb -e "external issue tracker"!

Outputs

Before

After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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.