config: eliminate hostnames config option.

Eliminate the Danbooru.config.hostnames option. It was only used for
rewriting links in notes. Just using the main hostname is good enough,
there aren't any notes still using any of Danbooru's alternate domain
names.

app/logical/note_sanitizer.rb

@@ -84,7 +84,7 @@ module NoteSanitizer
 
     url = Addressable::URI.heuristic_parse(node["href"]).normalize
 
-    if url.authority.in?(Danbooru.config.hostnames)
+    if url.authority == Danbooru.config.hostname
       url.site = nil
       node["href"] = url.to_s
     end