Remove deprecated jcalendar

Using latest Smarty, we get a fatal:

Smarty\CompilerException: "Syntax error in template "file:CRM/common/jcalendar.tpl" on line 41 "{assign var='displayDate' value=$elementId|cat:"_display"|cat:"_$string"|uniqid}" unknown modifier 'uniqid'"

Recommendation from core is to use:

$this->add('datepicker', 'start_date', ts('Campaign Start Date'), [], FALSE, ['time' => FALSE]);