Bug 1717225 - Remove superfluous border rule to restore the Bookmarks Menu Button menupopup border on Arch. r=mhowell

This rule was first introduced in bug 1700194 to get rid of a double-border
that was appearing on Windows 10.

I think bug 1708735 made this workaround unnecessary, and also results
in the border being missing on some distributions of Linux.

Removing this rule doesn't cause the double-border to recur on Windows 10,
and doesn't change the appearance on Windows 7, macOS or Ubuntu, so if
this fixes things for those Linux distributions, that's great.

Differential Revision: https://phabricator.services.mozilla.com/D119916

browser/themes/shared/customizableui/panelUI.inc.css

@@ -244,10 +244,6 @@ panelview .toolbaritem-combined-buttons,
   max-width: @standaloneSubviewWidth@;
 }
 
-#BMB_bookmarksPopup::part(arrowscrollbox) {
-  border: 0;
-}
-
 #BMB_bookmarksPopup menupopup {
   --panel-border-radius: var(--arrowpanel-border-radius);
   --panel-padding: var(--panel-subview-body-padding);