Drop deprecated GtkScrollbar-min-slider-length

It seems to serve no purpose anymore + added bonus:
Avoid deprecation warning littering journald logs. 

data/cheese.css

@@ -4,6 +4,5 @@ GtkScrollbar#vscrollbar {
   -GtkScrollbar-has-secondary-backward-stepper: 0;
   -GtkScrollbar-has-forward-stepper: 0;
   -GtkScrollbar-has-secondary-forward-stepper: 0;
-  -GtkScrollbar-min-slider-length: 60;
 }