* contrib/lisp/org-export.el (org-export-define-backend): Update
docstring.
(org-export-special-keywords): Remove "MACRO".
(org-export--get-inbuffer-options, org-export--get-buffer-attributes):
Remove internal macro handling.
(org-export-as): Use external tool instead.
(org-export-expand-macro): Remove function.
* contrib/lisp/org-e-ascii.el (org-e-ascii-macro): Remove function.
* contrib/lisp/org-e-groff.el (org-e-groff-macro): Remove function.
* contrib/lisp/org-e-html.el (org-e-html-macro): Remove function.
* contrib/lisp/org-e-latex.el (org-e-latex-macro): Remove function.
* contrib/lisp/org-e-man.el (org-e-man-macro): Remove function.
* contrib/lisp/org-e-odt.el (org-e-odt-macro): Remove function.
* contrib/lisp/org-e-texinfo.el (org-e-texinfo-macro): Remove
function.
* testing/lisp/test-org-export.el: Remove tests.
Since macros are now expanded before parsing (and produce Org syntax),
back-ends will never see them (unless NO-EXPAND argument from
`org-export-as' is non-nil, which isn't the case for any back-end so
far).