Delegate HSTS to nginx.

Remove the ssl_options config option. Let nginx handle HSTS and
http->https redirects instead. At the rails level, all we need to do is
set the secure cookie flag when https is enabled (which we assume it's
enabled in production).

config/application.rb

@@ -46,17 +46,6 @@ module Danbooru
 
     config.log_tags = [->(req) {"PID:#{Process.pid}"}]
     config.action_controller.action_on_unpermitted_parameters = :raise
-    config.force_ssl = true
-
-    if Rails.env.production? && Danbooru.config.ssl_options.present?
-      config.ssl_options = Danbooru.config.ssl_options
-    else
-      config.ssl_options = {
-        hsts: false,
-        secure_cookies: false,
-        redirect: { exclude: ->(request) { true } }
-      }
-    end
 
     if File.exist?("#{config.root}/REVISION")
       config.x.git_hash = File.read("#{config.root}/REVISION").strip