Former state Attorney General Gurbir Grewal dropped a political stink bomb during Sen. Bob Menendez’s corruption trial last week, when he testified that he had to fend off meddling inquiries into live criminal cases — not just from Menendez, but from former Sen. Dick Codey and from a former chief of staff for Gov. Murphy, George Helmy. Such acts contravene a rule that should be honored from the White House to your local municipal hall: Officeholders have no business calling a prosecutor to influence or even ask about an active investigation. These are, to borrow Grewal’s term, “Gross.” The in…