Fix crash when calling the planner twice in a row

Turns out that the fix in commit f7119bdccfb2 ("Planner: make sure no old
handles are around when entering Add/Plan") was incorrect. We ONLY want to
remove the existing handlers when we re-plan a dive, NOT when we call add
or plan.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>