Code Cleanup: Remove extraneous private variables

[funnylookinhat]

Rely solely on $this->_data when it comes to API functions. As an example, the following lines should not exist: https://github.com/system76/beansbooks/blob/master/application/classes/beans/vendor/purchase/invoice.php#L41-L48

They should be replaced with local values in _execute() that are parsed similarly.