ignoreheading tag does not work properly on level 4

[Berndone]

When using the ignoreheading tag it does not work on the first entry of a level 4 heading. E.g.

* A
** B
*** C
**** D1   :ignoreheading:
Foo
**** D2   :ignoreheading:
Bar

results in the exported block

\begin{enumerate}
\item D1\hfill{}\textsc{ignoreheading}
\label{sec:org0817d3c}
Foo
\label{sec:org5d8ea88}
Bar
\end{enumerate}

D1 gets exported, because the regex pattern in the function sa-ignore-headline in org-files-to-tex.el matches only the line \item D1\hfill{}\textsc{ignoreheading} but for the first entry \begin{enumerate} is part of the string which does not lead to a match. Extending the regex is not really an option: If \begin{enumerate} gets removed there is a lonely \end{enumerate}.

For me, replacing sa-ignore-headline with the following block worked, but I don't know if this has some unwanted side effects I'm not aware about:

(defun org-remove-headlines (backend)
  "Remove headlines with :ignoreheading: tag."
  (org-map-entries (lambda () (delete-region (point-at-bol) (point-at-eol)))
                   "ignoreheading"))
(add-hook 'org-export-before-processing-hook #'org-remove-headlines)

[fangohr]

Thanks for the input. I'd be happy to include a pull request with that change.