don't display js pop up on read-only date fields.

It's confusing to the users. See:
http://issues.civicrm.org/jira/browse/CRM-13565
fixing to match default smarty syntax.

http://issues.civicrm.org/jira/browse/CRM-13565
Merge pull request #1801 from jmcclelland/jscal-fix

No js cal on read only fields