HighContrast: No blue spinners

Spinners are drawing backgrounds now, so setting a blue
background color is pretty distracting.

gtk/theme/HighContrast/_common.scss

@@ -163,7 +163,6 @@ label {
 
 spinner {
   background-image: none;
-  background-color: blue;
   opacity: 0; // non spinning spinner makes no sense
   -gtk-icon-source: -gtk-icontheme('process-working-symbolic');
   &:checked {

gtk/theme/HighContrast/gtk.css

@@ -130,7 +130,6 @@ label:disabled, label:backdrop:disabled {
     -gtk-icon-transform: rotate(1turn); } }
 spinner {
   background-image: none;
-  background-color: blue;
   opacity: 0;
   -gtk-icon-source: -gtk-icontheme("process-working-symbolic"); }
   spinner:checked {