ox: Activate lexical binding

ox: Activate lexical binding

* lisp/ox.el (org-export-define-backend):
(org-export-define-derived-backend): Remove unused argument.

(org-export--get-inbuffer-options):
(org-export--list-bound-variables):
(org-export--selected-trees):
(org-export-transcoder):
(org-export--prune-tree):
(org-export--merge-external-footnote-definitions):
(org-export--footnote-reference-map):
(org-export-get-alt-title):
(org-export-get-node-property):
(org-export-table-row-is-special-p):
(org-export-table-dimensions):
(org-export-stack-refresh): Refactor code.

* testing/lisp/test-ox.el (org-test-default-backend):
(test-org-export/with-backend):
(test-org-export/footnote-first-reference-p): Comply to lexical binding.