EXT-803 Parcel highlight stays lit after setting a parcel for sale and closing...

EXT-803 Parcel highlight stays lit after setting a parcel for sale and closing the About Land floater.  The LLFloaterReg conversion made this floater single_instance, which means it does not destroy itself on close.  We therefore need to explicitly clean up the parcel selection when the sell land floater is closed.  Reviewed with Leyla.