Trackers that have parent_issue_id in their disabled_core_fields should not be selectable for new child issues
When the user views an issue and clicks the link to add a new child issue, and the project has some trackers where the
parent_issue_id field is disabled, the resulting new issue form for the child issue still presented those trackers for the user to choose from.
In case the tracker of the parent issue has
parent_issue_id disabled itself, a new child issue would even have that tracker pre-selected through the
tracker_id URL param, which is also fixed by the patch.
Patch extracted from Planio