Move `org-heading-delete-metadata' into "org-list.el"
* lisp/org.el (org-heading-delete-metadata): Move to...
* lisp/org-list.el (org-list--delete-metadata): ... here. Refactor
code.
(org-toggle-item): Use new function.
* testing/lisp/test-org-list.el (test-org-list/toggle-item): Add test.
At the moment, this function is only used internally in "org-list.el",
so it belongs there.
Also refrain from removing log or clock drawers, because they may not
contain all clocks or notes anyway. Besides, the information they
contain may still make sense in a plain list.