CRM-14930 - fix underlying data problem with authnet net_amount

CRM-14930 - fix underlying data problem with authnet net_amount

Revert "CRM-14881 - avoid error when saving contribution made by authnet"

This reverts commit b29eb7605df438ab222f1f741d220b145da2f059.

Conflicts:
	CRM/Contribute/Form/Contribution.php

Reverting in favor of a solution that solves the underlying data
problem, rather than a solution that works around a data problem.

The original issue is: https://issues.civicrm.org/jira/browse/CRM-14881
This revert is: https://issues.civicrm.org/jira/browse/CRM-14930