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.

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))