org-element: Disable cache by default

* lisp/org-element.el (org-element-use-cache): Set default value to
  nil.

Using cache introduces Emacs' freezes under not well known
circumstances.  Until this is sorted out, this variable should be nil,
at least on "maint" branch.

lisp/org-element.el

@@ -4804,9 +4804,12 @@ indentation removed from its contents."
 ;; `org-element--cache-objects'.
 
 
-(defvar org-element-use-cache t
-  "Non nil when Org parser should cache its results.
-This is mostly for debugging purpose.")
+(defvar org-element-use-cache nil
+  "Non-nil when Org parser should cache its results.
+
+WARNING: for the time being, using cache sometimes triggers
+freezes.  Therefore, it is disabled by default.  Activate it if
+you want to help debugging the issue.")
 
 (defvar org-element-cache-sync-idle-time 0.6
   "Length, in seconds, of idle time before syncing cache.")