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

config/danbooru_default_config.rb

@@ -34,12 +34,6 @@ module Danbooru
       Socket.gethostname
     end
 
-    # The list of all domain names this site is accessible under.
-    # Example: %w[danbooru.donmai.us sonohara.donmai.us hijiribe.donmai.us safebooru.donmai.us]
-    def hostnames
-      [hostname]
-    end
-
     # Contact email address of the admin.
     def contact_email
       "webmaster@#{server_host}"

test/unit/note_sanitizer_test.rb

@@ -23,7 +23,7 @@ class NoteSanitizerTest < ActiveSupport::TestCase
     end
 
     should "rewrite absolute links to relative links" do
-      Danbooru.config.stubs(:hostnames).returns(%w[danbooru.donmai.us sonohara.donmai.us hijiribe.donmai.us])
+      Danbooru.config.stubs(:hostname).returns("sonohara.donmai.us")
 
       body = '<a href="http://sonohara.donmai.us/posts?tags=touhou#dtext-intro">touhou</a>'
       assert_equal('<a href="/posts?tags=touhou#dtext-intro" rel="external noreferrer nofollow">touhou</a>', NoteSanitizer.sanitize(body))