Delete work item project.fullPath

What does this MR do and why?

Delete work item project.fullPath

Previously, we could view any work item at route /gitlab-org/gitlab-test/-/work_items/<id>. However, the work item could be part of a project outside of gitlab-org/gitlab-test since we were getting the details by ID, so we updated code to get the work item's project.fullPath so we could fetch the assignees etc from the correct project.

Now, we fetch by IID at /gitlab-org/gitlab-test/-/work_items/<iid> so we always view a work item within the same project. As a result, we no longer need project.fullPath

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