Let DialogObjectForm do the implicit isCreate handling

To work this needs: !1798
Otherwise it would not work in normal CRUD-context.

aleksis/core/frontend/components/calendar/CalendarEventDialog.vue

@@ -18,7 +18,6 @@ import GroupField from "../generic/forms/GroupField.vue";
     :get-patch-data="filterData"
     :default-item="defaultItem"
     :edit-item="editItemWithDatetimes"
-    :is-create="!editItem"
     :fields="editableFields"
     :minimal-patch="false"
   >