[GeckoView] Remove unused transition CSS-rules

Given that the GeckoView-viewer doesn't have a sidebar, there's no reason to have CSS-rules for it (and the variables are also undefined).

web/viewer-geckoview.css

@@ -94,10 +94,6 @@ body {
   inset: 0;
   outline: none;
 }
-#viewerContainer {
-  transition-duration: var(--sidebar-transition-duration);
-  transition-timing-function: var(--sidebar-transition-timing-function);
-}
 
 .dialogButton {
   border: none;