Remove unneccessary isoToDate function

[CQ] Let's celebrate the recent 6 year anniversary of making isoToData unnecessary by getting it out of the code

Prior to July 2014 the following would fail

$contribution->fetch();
$contribution->save();

because the fetch format for dates was invalid for save.

However, that all changed once this was merged https://github.com/civicrm/civicrm-packages/commit/283da6111677fc2b0176902b4c9a5cbcf668a258

Except for the bits that provide handling for it still litter our code