* lisp/arc-mode.el: Remove unused struct fields

* lisp/arc-mode.el: Remove unused struct fields

(archive--file-desc): Remove `case-fiddled`.
Change all subtypes's constructors as their callers accordingly.
(archive--file-desc-case-fiddled): New function.
(archive-int-to-mode): Accept a nil input.
Make all callers take advantage of it.
(archive-arc-rename-entry): Use `make-string`.
(archive-zip--file-desc): Change `pos+len` field into `pos` field.
(archive-zip-chmod-entry): Simplify accordingly.
(archive-zip-summarize): Don't bother with `lheader` which was not used.
(archive-zoo--file-desc): Delete struct; use archive--file-desc instead.
(archive-7z--file-desc): Remove `user` and `group` fields.
Adjust constructor and its caller.
(archive-ar-summarize): Use `archive-int-to-mode`.