Remove unnecessary linking to export framework

Remove unnecessary linking to export framework

* lisp/org-clock.el (org-clocktable-defaults,
  org-clocktable-write-default): Avoid requiring the whole export
  framework just to check one variable.
* lisp/org-footnote.el (org-footnote-section): Update
  docstring.
(org-footnote-normalize): Remove all export related part from the
  function.
* lisp/org-inlinetask.el (org-inlinetask-export,
  org-inlinetask-export-templates): Removed variables.
(org-inlinetask-export-handler): Removed function.
* lisp/org-plot.el: Remove dependency on `org-exp' library.
* lisp/org.el (org-additional-option-like-keywords): Remove variable.
(org-get-export-keywords): New function.
(org-options-keywords): Update default list of keywords.
(org-remove-flyspell-overlays-in): Apply changes to keywords
compilation.
(org-highlight-latex-fragments-and-specials)
org-latex-and-specials-regexp,
org-export-html-special-string-regexps): Remove variables.
(org-compute-latex-and-specials-regexp,
org-do-latex-and-special-faces, org-remove-file-link-modifiers):
Remove functions.
(org-set-regexps-and-options, org-set-font-lock-defaults): Apply all
removals.
(org-use-sub-superscripts): Fix docstring.  Remove unused group.
(org-match-sexp-depth): Make it a defconst instead of a defcustom in
order to remove `org-export-translation' group completely.

The new export framework shouldn't be as much interleaved with Org
core as with the previous one.  For example, export options, which are
back-end dependent, shouldn't interfere with the display of the
buffer.