Clean up.

Clean up.
(nnshimbun-nov-last-check): Removed.
(nnshimbun-nov-buffer-alist): Removed.
(nnshimbun-nov-buffer-file-name): Removed.
(nnshimbun-close-server): Use `nnshimbun-write-nov' instead of
`nnshimbun-save-nov'.
(nnshimbun-mail-header-subject): Removed.
(nnshimbun-mail-header-from): Removed.
(nnshimbun-make-shimbun-header): Removed.
(nnshimbun-parse-nov): New function.
(nnshimbun-request-article-1): Use `nnshimbun-parse-nov' instead
of `nnheader-parse-nov'.
(nnshimbun-retrieve-headers): Likewise.
(nnshimbun-nov-buffer-name): New function.
(nnshimbun-nov-file-name): Ditto.
(nnshimbun-open-nov): Clean up.
(nnshimbun-write-nov): Ditto; Aceept the 2nd optional argument.
(nnshimbun-save-nov): Removed.