org-element: Remove :inline-definition from inline footnotes

org-element: Remove :inline-definition from inline footnotes

* lisp/org-element.el (org-element-recursive-objects): Add
  `footnote-reference'.
(org-element-secondary-value-alist): Remove reference to
`footnote-reference'.
(org-element-footnote-reference-parser): Definition for inline
references is stored as the contents of the reference, not in
a secondary string.
(org-element-footnote-reference-interpreter): Apply changes from
parser.

* lisp/ox.el (org-export-get-footnote-definition,
  org-export-get-environment): Apply changes from parser.

* testing/lisp/test-org-element.el (test-org-element/footnote-reference-parser):
  Update test.
(test-org-element/context): Add test.

Storing definition in a secondary string was a poor design choice as
there is no "primary" string anyway.  This also prevents
`org-element-context' from finding objects within the inline
definition.