org-element: Remove folding status in parsed data
* lisp/org-element.el (org-element-center-block-parser,
org-element-drawer-parser, org-element-dynamic-block-parser,
org-element-item-parser, org-element-quote-block-parser,
org-element-comment-block-parser, org-element-export-block-parser,
org-element-verse-block-parser, org-element-special-block-parser,
org-element-example-block-parser, org-element-headline-parser,
org-element-inlinetask-parser): Remove :hiddenp property.
* lisp/org.el (org-end-of-line, org-down-element): Use an equivalent
of :hiddenp property.
* testing/lisp/test-org-element.el: Remove tests.
The property is removed because it is buggy (e.g., when there's a link
just after a block opening line) and not really useful.