[Beta] Remove nav fadein

It's pretty distracting when switching tabs.

beta/src/components/Layout/Sidebar/SidebarRouteTree.tsx

@@ -64,7 +64,6 @@ function CollapseWrapper({
       className={cn(isExpanded ? 'opacity-100' : 'opacity-50')}
       style={{
         transition: `opacity ${duration}ms ease-in-out`,
-        animation: `nav-fadein ${duration}ms ease-in-out`,
       }}>
       <div {...getCollapseProps()}>{children}</div>
     </div>

beta/src/styles/index.css

@@ -153,15 +153,6 @@
     display: none;
   }
 
-  @keyframes nav-fadein {
-    from {
-      opacity: 0.5;
-    }
-    to {
-      opacity: 1;
-    }
-  }
-
   /*
    * Hopefully when scrollbar-color lands everywhere,
    * (and not just in FF), we'll be able to keep just this.