Bug 1868490 - Remove fixed width from toolbarseparators on Linux. r=desktop-theme-reviewers,dao

Before bug 1866022, the default native appearance of these on Linux was
for a GTK toolbar separator, which is a separator for the horizontal
items of a toolbar.

However it seems we use them as regular vertical separators everywhere,
using actual <spacer> elements for the horizontal separators.

So remove the hard-coded width, which causes this bug.

Differential Revision: https://phabricator.services.mozilla.com/D195633

toolkit/themes/linux/global/toolbar.css

@@ -32,7 +32,6 @@ toolbar {
 }
 
 toolbarseparator {
-  width: 12px;
   min-width: 2px;
 }