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
CRM-14930 - net amount and fee amount are not provided by authnet

Removing these lines because they will never be set (which should allow
the values to be set to NULL anyway).

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* CRM-14930: Authorize.net silentpost transactions cannot be edited due to invalid net_amount
  https://issues.civicrm.org/jira/browse/CRM-14930
CRM-14930 - ensure data integrity of all contributions.

Some auth.net contributions may  not have correct net amount set.

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* CRM-14930: Authorize.net silentpost transactions cannot be edited due to invalid net_amount
  https://issues.civicrm.org/jira/browse/CRM-14930
Merge pull request #3871 from jmcclelland/final-fix-invalid-net-amount

Fixing data integrity problem with net amount and auth.net