DEV: Remove handling of category top menu items

Support for these kinds of navigation items was dropped in 88f52514, but
the code for handling these menu items was never removed.

test/javascripts/fixtures/site-fixtures.js.es6

@@ -41,7 +41,6 @@ export default {
         "starred",
         "read",
         "posted",
-        "category",
         "categories",
         "top"
       ],
@@ -49,7 +48,6 @@ export default {
         "latest",
         "top",
         "categories",
-        "category",
         "categories",
         "top"
       ],