Prevent VehicleEnterEvent being sent when player exits vehicle.

Fix for BUKKIT-223.

Issue BUKKIT-223: When a player exits a minecart or boat, both a
VehicleExitEvent and a VehicleEnterEvent are fired.  Only the
VehicleExitEvent should fire.

Reason for bug: This occurs because the VehicleEnterEvent is fired in
EntityBoat.b and EntityMinecart.b *any* time a player right-clicks on
a vehicle, whether the right-click is to enter the vehicle or exit it.

Fix: By moving the creation of VehicleEnterEvents from EntityBoat.b
and EntityMinecart.b to Entity.setPassengerOf, we can create either a
VehicleEnterEvent or a VehicleExitEvent, depending on whether the
player is entering or exiting a vehicle.

src/main/java/net/minecraft/server/EntityBoat.java

@@ -420,15 +420,6 @@ public class EntityBoat extends Entity {
             return true;
         } else {
             if (!this.world.isStatic) {
-                // CraftBukkit start
-                VehicleEnterEvent event = new VehicleEnterEvent((Vehicle) this.getBukkitEntity(), entityhuman.getBukkitEntity());
-                this.world.getServer().getPluginManager().callEvent(event);
-
-                if (event.isCancelled()) {
-                    return true;
-                }
-                // CraftBukkit end
-
                 entityhuman.mount(this);
             }