fix: z-index clash in mobile UI (#8985)

packages/excalidraw/components/FixedSideContainer.scss

@@ -15,7 +15,6 @@
     top: var(--editor-container-padding);
     right: var(--editor-container-padding);
     bottom: var(--editor-container-padding);
-    z-index: 2;
   }
 
   .FixedSideContainer_side_top.zen-mode {