[C] Add VehicleExitEvent when leaving Entity. Fixes: BUKKIT-3761

Description:

The Issue:

VehicleExitEvent not triggered when leaving entity.

Justification for this PR:

Many plugins use this event for improving server functionality. Without a fix the event will not function as intended to.

PR Breakdown:

PR is reverting some of the new changes to Entity and EntityHuman and resolve problem where VehicleExitEvent should be triggered when this.vehicle == entity.

JIRA Ticket:

https://bukkit.atlassian.net/browse/BUKKIT-3761