Remove Sisimai::Message.load method and other related codes

[azumakuniyuki]

• The followings will be removed at Sisimai 5.1.1
  • Sisimai::Message.load
    • https://github.com/sisimai/rb-sisimai/blob/5-stable/lib/sisimai/message.rb#L109-L148
  • Code for calling Sisimai::Message.load
    • https://github.com/sisimai/rb-sisimai/blob/5-stable/lib/sisimai/message.rb#L40-L48
    • https://github.com/sisimai/rb-sisimai/blob/5-stable/lib/sisimai/message.rb#L425-L432
  • Other related codes in test/public/message-test.rb
• Does anyone call Sisimai::Message.load?
  • Does anyone specify load or order parameter at the argument of Sisimai::Message.rise() method or Sisimai.rise() method?