This fixes an issue where pressing Shift + Up/Down while focused on the
overflow button will result in a bail out when asserting the focused
widget has a month cell ancestor:
```
GcalMonthView:ERROR:../src/gui/views/gcal-month-view.c:1002:on_key_controller_key_pressed_cb: assertion failed: (GCAL_IS_MONTH_CELL (month_cell))
Bail out! GcalMonthView:ERROR:../src/gui/views/gcal-month-view.c:1002:on_key_controller_key_pressed_cb: assertion failed: (GCAL_IS_MONTH_CELL (month_cell))
```
When navigating between month cells with a keyboard, focus is set
on respective month cells, so checking whether the ancestor
is a month cell or not is not only unnecessary, it also causes
bugs like the assertion above.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/720>