Removing obsolete function `org-truely-invisible-p'.

[Karl Fogel <kfogel@red-bean.com>]

The function `org-truely-invisible-p' is defined in 
'lisp/org-macs.el', but it is not used anywhere anymore in Org 
Mode, nor is it used anywhere in GNU Emacs (I checked on both 
'emacs-28' branch and 'master' branch).

The last (and possibly only?) call to that function was removed 
from `org-beginning-of-line' in this commit:

  commit 3baf246f4f73005a4eacd7c368f7222f95d50243
  Author:     Nicolas Goaziou <mail@nicolasgoaziou.fr>
  AuthorDate: Thu Apr 28 23:28:15 2016 +0200
  Commit:     Nicolas Goaziou <mail@nicolasgoaziou.fr>
  CommitDate: Thu Apr 28 23:28:15 2016 +0200
  
      Handle correctly `shift-select-mode'
      
      * lisp/org.el (org-beginning-of-line): Handle correctly 
      `shift-select-mode'.
      
      Reported-by: Mathieu Marques <mathieumarques78@gmail.com>
      <http://permalink.gmane.org/gmane.emacs.orgmode/106783>

Then, a few months later in commit 87116700e6e, Nicholas moved the 
function to 'org-macs.el', where it has been sitting unused ever 
since.

Should we just remove `org-truely-invisible-p'?  Or at least 
correct the spelling of its name ("truely" should be "truly")?

It seems fairly unlikely to me that people are using it in their 
own code, although of course I cannot be sure of that.  Partly, I 
believe that because surely someone else would have noticed the 
misspelling by now, if enough people were using it :-).

Best regards,
-Karl