remove uses of -moz-proton media query

chrome/navbar_tabs_oneliner.css

@@ -19,9 +19,10 @@ urlbar_popup_full_width.css is VERY MUCH recommended for Firefox 71+ because of
   --uc-toolbar-height: 40px;
   --uc-urlbar-min-width: 50vw; /* minimum width for opened urlbar */
 }
-@media (-moz-proton){
-  #scrollbutton-up,#scrollbutton-down{ border-block-width: 2px !important; }
-}
+
+#scrollbutton-up,
+#scrollbutton-down{ border-block-width: 2px !important; }
+
 /* Override for other densities */
 :root[uidensity="compact"] #navigator-toolbox{ --uc-toolbar-height: 34px; }
 :root[uidensity="touch"] #navigator-toolbox{ --uc-toolbar-height: 44px; }