org-element: Remove :alt-title property for headlines
* lisp/org-element.el (org-element-headline-parser): Remove :alt-title.
* lisp/ox.el (org-export-get-alt-title): Check regular :ALT_TITLE
property instead of :alt-title.
`:alt-title' is an export-specific property. As a consequence, it's
value doesn't need to be parsed at "org-element.el"'s level. There, it
is treated as a regular property : `ALT_TITLE'.
This also removes the last call to
`org-element-parse-secondary-string' from "org-element.el".