contrib/lisp/org-export: Fix headline numbering

contrib/lisp/org-export: Fix headline numbering

* contrib/lisp/org-export.el (org-export-persistent-properties-list):
  Remove unused persistent properties.
(org-export-initialize-persistent-properties): Delegate headline
  numbering alist to `org-export-collect-headline-numbering'.
(org-export-collect-headline-numbering): New function.
(org-export-update-info): Remove part relative to headline numbering.
(org-export-get-headline-number): Use new `:headline-numbering' to get
  current headline number.

Headline numbering is not done on demand anymore, but built once and
for all at the beginning of the export process, and stored in an
alist, accessible through the `:headline-numbering' property in the
communication channel.