scaladoc: delete color-brewer.css (scala#16606)

`color-brewer.css` is commented out in
scala@3474830.
So, it is unnecessary. And we can reduce the http request if we delete
it from `base.html`.

Thank you :)

Author: szymon-rd

docs/_assets/css/color-brewer.css

@@ -1,7 +1,3 @@
----
-extraCSS:
-    - css/color-brewer.css
----
 <link rel="dns-prefetch" href="//fonts.googleapis.com">
 <link rel="preconnect" href="https://cdnjs.cloudflare.com" crossorigin>
 <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>