Fix Interactive2.MovablePoint.remove()

Summary:
since MovablePoint.modify() naively calls this.remove()
then this.add() right now, and this.remove() did not
set visibleShape or mouseTarget to null, the point
would "modify" the removed visibleShape and mouseTarget
instead of creating new ones, causing the movablePoint
to disappear.

Test Plan:
Make a movablePoint in a <Graphie> component (which calls
modify) and make sure you can drag the point around
multiple times (note: <Graphie> isn't committed yet, but
that's how this was tested)

Auditors: justinjaffray, brianmerlob