Remove unused option None for LEVEL_FILTERS_DEFAULT (#15315)

Take to account that we don't have lints under under `None` for
`LEVEL_FILTERS_DEFAULT` so far
https://rust-lang.github.io/rust-clippy/master/index.html?levels=none,
we can remove it.

changelog: none

Author: Alexendoo

util/gh-pages/script.js

@@ -208,7 +208,6 @@ const LEVEL_FILTERS_DEFAULT = {
     allow: true,
     warn: true,
     deny: true,
-    none: true,
 };
 const APPLICABILITIES_FILTER_DEFAULT = {
     Unspecified: true,