CRM-12417 format activity date during import

format activity date during import; this reverts:
https://fisheye2.atlassian.com/browse/~br=trunk/CiviCRM/trunk/CRM/Activity/Import/Parser/Activity.php?r2=45745&r1=45739

which was an unnecessary change and introduced a regression bug (http://issues.civicrm.org/jira/browse/CRM-11904)