Drop hashcash support from Gnus and Message mode

Drop hashcash support from Gnus and Message mode

* lisp/gnus/gnus.el (gnus-install-group-spam-parameters): Remove
'spam-use-hashcash' option.
* lisp/gnus/message.el (message-generate-hashcash): Mark as
obsolete, default to nil unconditionally.
(message-send-mail, message-setup-1, message-resend): Don't
generate hashcash headers.
* lisp/gnus/spam.el (hashcash): Don't require.
(spam-use-hashcash):
(spam-check-hashcash): Mark as obsolete.
(spam-install-hooks): Don't install 'spam-use-hashcash' hook.
* doc/misc/message.texi (Mail Headers): Don't document above
obsoleted variable 'message-generate-hashcash'.
* doc/misc/gnus.texi (Hashcash): Delete section.
(Anti-spam Hashcash Payments): Delete subsection.
(Extending the Spam package): Don't mention Hashcash.  (Bug#76195)

(cherry picked from commit 250f4214752d223981259e453c55a8c5cabb111a)