org-src: Remove unused functions and variables
* lisp/org-src.el (org-edit-src-region-extra): Remove variable.
(org-edit-src-find-region-and-lang, org-edit-src-get-lang,
org-edit-src-get-label-format, org-edit-src-get-indentation,
org-src-fontify-block, org-src-fontify-buffer): Remove functions.
(org-src-font-lock-fontify-block): Remove reference to removed
function in docstring.
Remove all functions and variables related to parsing, which is now
delegated to org-element.el. Also reorder a defcustom and
a declaration in the library.