org-element: Remove `org-element-document-properties'
* lisp/org-element.el (org-element-document-properties): Remove
variable.
(org-element-context): Ignore ex-"document keywords".
* testing/lisp/test-org-element.el (test-org-element/context): Remove
no longer applicable tests.
Parsed keywords are no longer hard-coded and can be set per export
back-end.
As a consequence, `org-element-context' no longer returns objects in
"TITLE", "DATE" or "AUTHOR" keywords. This makes sense as these
objects would only make sense during export, and only if the chosen
back-end actually uses them.