Fix datetime format issue with readonly attribute.

Corresponding to tracker_item_id=30244 (http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=30244).

I replaced the % in format(specified by user) of calender with null string("") so that PHP can format the output datetime according to format. This is happening only when user specified a read only or disabled attribute in the xml file.