Remove override of the Framework's fireModelEvent method

[kfriars]

This change is motivated by 2 factors:

1. Reduce the likelihood of breaking changes being introduced by the framework
2. Allow other packages and applications to rely on the standard interface of the fireModelEvent method

This pull request does not change any functionality or behavior of the package. The only breaking change I could foresee is if a consuming application has started firing other model events with the custom fireModelEvent payload arguments. In that case, they should update their invocations of fireModelEvent to firePivotEvent.