gems: drop Mechanize, HTTParty, and Sinatra gems.

config/danbooru_default_config.rb

@@ -340,22 +340,6 @@ module Danbooru
     def twitter_api_secret
     end
 
-    # The default headers to be sent with outgoing http requests. Some external
-    # services will fail if you don't set a valid User-Agent.
-    def http_headers
-      {
-        "User-Agent" => "#{Danbooru.config.canonical_app_name}/#{Rails.application.config.x.git_hash}"
-      }
-    end
-
-    def httparty_options
-      # proxy example:
-      # {http_proxyaddr: "", http_proxyport: "", http_proxyuser: nil, http_proxypass: nil}
-      {
-        headers: Danbooru.config.http_headers
-      }
-    end
-
     # you should override this
     def email_key
       "zDMSATq0W3hmA5p3rKTgD"