Change pln_dropoff() parameters to match pln_newlanding()

This moves getting the target sector or ship from caller into
pln_dropoff().  Makes sense, because that's where it's put.

src/lib/commands/drop.c

@@ -133,7 +133,7 @@ drop(void)
 	if (wantflags & P_MINE)
 	    pln_mine(&bomb_list, &target);
 	else
-	    pln_dropoff(&bomb_list, ip, tx, ty, &target, EF_SECTOR);
+	    pln_dropoff(&bomb_list, ip, tx, ty, -1);
     }
     pln_put(&bomb_list);
     pln_put(&esc_list);