Clear deprecated user prefs (#390)

plugins/calendar/calendar.php

@@ -468,6 +468,8 @@ class calendar extends rcube_plugin
         'calendar_default_alarm_type'   => get_input_value('_alarm_type', RCUBE_INPUT_POST),
         'calendar_default_alarm_offset' => $default_alam,
         'calendar_default_calendar'     => get_input_value('_default_calendar', RCUBE_INPUT_POST),
+        'calendar_date_format' => null,  // clear previously saved values
+        'calendar_time_format' => null,
       );
 
       // categories