Bug 1560509 - Remove unneeded rule from panelUI.css. r=pmorris DONTBUILD

mail/themes/shared/customizableui/panelUI.css

@@ -1140,10 +1140,6 @@ panelview .toolbarbutton-1,
   transform: translateY(1px);
 }
 
-#main-window:not([customizing]) .subviewbutton-nav[disabled=true]::after {
-  opacity: 0.4;
-}
-
 .PanelUI-subView .subviewbutton-nav:-moz-locale-dir(ltr)::after {
   transform: scaleX(-1) translateY(1px);
 }