Revert "BS-8730: add foreign key definition between flownnode_instance and process_instance to improve database consistency"

This reverts commit cf5b1ed53715c6e251054d95141ed9e6b4172db8.

We have found that we cannot add a foreign key between this tables because in SP, when deleting a flownode, it flags it as 'deleted', but when deleting a processinstance, it does not flag it nor it checks the existence of flownode_instance