remove --proton-tab-block-margin dummy variables

chrome/autohide_tabstoolbar.css

@@ -3,9 +3,6 @@ See the above repository for updates as well as full license text. */
 
 /* Hide tabs unless cursor is on top of toolbar */
 
-/* Dummy variable to support both versions 89-90 */
-:root{ --tab-block-margin: var(--proton-tab-block-margin) }
-
 :root:not([customizing]) #navigator-toolbox{
   position: relative;
   z-index: 1;