left_sidebar: Remove z-index from sidebar-search.

Now that the left-sidebar filter sits atop the area,
it's no longer necessary to set a z-index here.

This also fixes a bug where a hovered unread in the
collapsed views would be clipped by the filter box
above.

web/styles/left_sidebar.css

@@ -338,8 +338,6 @@
         auto;
     position: sticky;
     top: 0;
-    /* Must be more than .stream-list-subsection-header */
-    z-index: 3;
     background: var(--color-background);
     /* Must be padding not margin so that the sticky headers don't show behind it */
     padding: 0 var(--left-sidebar-right-margin) 3px 5px;