Remove backdrop-filter from docs subnav

Fixes twbs#31163.

Author: mdo

site/assets/scss/_subnav.scss

@@ -2,8 +2,7 @@
   // The position and z-index are needed for the dropdown to stay on top of the content
   position: relative;
   z-index: $zindex-sticky;
-  background-color: rgba($white, .75);
-  backdrop-filter: blur(1rem);
+  background-color: rgba($white, .95);
   box-shadow: 0 .5rem 1rem rgba(0, 0, 0, .05), inset 0 -1px 0 rgba(0, 0, 0, .15);
 
   .dropdown-menu {