org-element: Ignore plain footnotes

org-element: Ignore plain footnotes

* lisp/org-element.el (org-element--set-regexps):
(org-element-footnote-definition-interpreter):
(org-element-footnote-reference-parser):
(org-element-footnote-reference-interpreter): Do not consider [1]-like
constructs as footnotes anymore.

* lisp/ox.el (org-export-expand-include-keyword):
(org-export--prepare-file-contents): Apply changes to footnotes.

* doc/org.texi (Footnotes): Remove references to plain footnotes.

* testing/lisp/test-org-element.el (test-org-element/footnote-reference-parser):
(test-org-element/footnote-reference-interpreter):
* testing/lisp/test-ox.el (test-org-export/expand-include):
(test-org-export/expand-macro):
(test-org-export/get-footnote-number):
(test-org-export/collect-footnote-definitions):
(test-org-export/footnotes):
(test-org-export/fuzzy-link): Update tests.

Since it is possible to refer unambiguously to a label without the "fn:"
prefix, the latter becomes part of the syntax and no longer part of the
label.  In particular [fn:1] and [fn:label] are labelled, respectively,
"1" and "label".