Remove alert for Internet Explorer 8 and below

Internet Explorer 8 was released in 2009 and people using it already
know that most web pages look broken on it, so we don't need to warn
them.

Removing it makes our application layout file much easier to read and
modify.

app/assets/stylesheets/layout.scss

@@ -223,17 +223,6 @@ button,
   color: $check;
 }
 
-.ie-callout {
-  position: absolute;
-  top: 0;
-  width: 100%;
-
-  .close {
-    font-size: rem-calc(34);
-    top: 20%;
-  }
-}
-
 .menu.simple {
   border-bottom: 1px solid $border;
   clear: both;