org.el: obsolete org-in-fixed-width-region-p

* lisp/org.el (org-in-fixed-width-region-p): Make obsolete.

It has no calls in core or contrib, and its function would be better
served by using the parser.

lisp/org-compat.el

@@ -187,6 +187,8 @@ Counting starts at 1."
 (define-obsolete-variable-alias 'org-html-style 'org-html-head "24.4")
 (define-obsolete-function-alias 'org-insert-columns-dblock
   'org-columns-insert-dblock "Org 9.0")
+(make-obsolete 'org-in-fixed-width-region-p "the `org-element' library"
+	       "Org 9.0")
 
 (defcustom org-read-date-minibuffer-setup-hook nil
   "Hook to be used to set up keys for the date/time interface.

lisp/org.el

@@ -20960,7 +20960,9 @@ See the individual commands for more information."
     (org-paste-subtree arg)))
 
 (defsubst org-in-fixed-width-region-p ()
-  "Is point in a fixed-width region?"
+  "OBSOLETE
+
+Is point in a fixed-width region?"
   (save-match-data
     (eq 'fixed-width (org-element-type (org-element-at-point)))))