gnome-shell/calendar: do not change color of day with events
When a non-working day has an event, the color is different from other non-working days without a date. This happens because the color of days with events is set explicitly, this might be wanted, but the view looks uneven this way.
To prevent this, do not explicitly setting a specific color for days with events. The color is expected to stay the same regardless the day being a working or non-working one.
Fixes: #2588 (closed)