Do not disable current day of week checkbox for weekly recurrence

There is code in the incidence recurrence UI which disables one day of the week for the weekly recurrence checkbox combobox, meaning you can't check or uncheck it.

This doesn't make any sense to me. If you're creating a recurring event, it probably should default to selecting the current day of the week. However there is never any reason to prevent selecting/deselecting some particular day. This can make it impossible to edit your weekly event in the way you want.

This MR removes the code which disables a day by default.

Chesterton's fence applies - this is old code, clearly put there intentionally, but I can't see why it's there. Feedback welcome!