Remove need to declare drawers before using them

Remove need to declare drawers before using them

* lisp/org-agenda.el (org-agenda-prepare): Remove reference to
  `org-drawers-for-agenda'.
(org-agenda-show-1): Remove reference to `org-drawers'.
* lisp/org-clock.el (org-clock-remove-empty-clock-drawer,
  org-clock-cancel): Apply signature change to
  `org-remove-empty-drawer-at'.
* lisp/org-element.el (org-element--list-struct): Use
  `org-drawer-regexp' instead of `org-drawers'.
* lisp/org-feed.el (org-feed-drawer): Update docstring according to
  change.
* lisp/org-list.el (org-in-item-p, org-list-context): Use
  `org-drawer-regexp' instead of `org-drawers'.
* lisp/org-mobile.el (org-mobile-create-index-file): Remove reference
  to `org-drawers-for-agenda'.
* lisp/org-pcomplete.el (pcomplete/org-mode/drawer): Remove function.
* lisp/org.el (org-drawer-regexp): Make variable global.
(org-drawers): Remove variable.
(org-set-regexps-and-options): Ignore DRAWER keyword.
(org-cycle): Use `org-drawer-regexp' instead of `org-drawers'.
(org-cycle-hide-drawers): Add an optional argument to ignore some
drawers.
(org-remove-empty-drawer-at): Remove second argument.  Rewrite
function.
(org-clone-subtree-with-time-shift): Apply signature change to
`org-remove-empty-drawer-at'.
(org-toggle-ordered-property): Apply `org-delete-property' signature
change.
(org-map-entries): Remove reference to `org-drawers-for-agenda'.
(org-entry-delete): Remove optional argument.  Small refactoring.
(org-insert-drawer): Remove reference to `org-drawers'.
(org-delete-property): Apply `org-entry-delete' signature change.
(org-in-drawer-p): Rewrite function.
* testing/lisp/test-org-element.el: Update tests.
* testing/lisp/test-org.el: Update tests.
* testing/lisp/test-ox.el: Update tests.