Revert "Give better errors in signing failures in Gnus"

This reverts commit ef2059e877.

Apparently this test triggered both when signing was successful
and unsuccessful (bug#26298).

lisp/gnus/mml1991.el

@@ -275,8 +275,6 @@ Whether the passphrase is cached at all is controlled by
 	(mm-decode-content-transfer-encoding cte)))
     (let* ((pair (mml-secure-epg-sign 'OpenPGP 'clear))
 	   (signature (car pair)))
-      (unless (stringp signature)
-        (error "Signature failed"))
       (delete-region (point-min) (point-max))
       (insert
        (with-temp-buffer

lisp/gnus/mml2015.el

@@ -958,8 +958,6 @@ If set, it overrides the setting of `mml2015-sign-with-sender'."
     (let* ((pair (mml-secure-epg-sign 'OpenPGP t))
 	   (signature (car pair))
 	   (micalg (cdr pair)))
-      (unless (stringp signature)
-        (error "Signature failed"))
       (goto-char (point-min))
       (insert (format "Content-Type: multipart/signed; boundary=\"%s\";\n"
 		      boundary))