views/month-cell: Remove dimming of out-of-month cells

The infinite scrolling month view obsoletes the out-of-month
dimming, as the view is no longer fixed in a specific month, nor
does it make out-of-month cells insensitive.

Closes: https://gitlab.gnome.org/GNOME/gnome-calendar/-/issues/1145
Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/646>

src/theme/style.css

@@ -275,11 +275,6 @@ monthcell:hover:selected {
     color-mix(in srgb-linear, var(--bg-color) 77.5%, var(--accent-bg-color));
 }
 
-/* Out of Month */
-monthcell.out-of-month > * {
-  opacity: calc(var(--dim-opacity) - 17.5%);
-}
-
 monthcell:selected label.day-label {
   font-weight: bold;
 }