org.el (org-time-clocksum-use-effort-durations): Use nil as the default

* org.el (org-time-clocksum-use-effort-durations): Don't set to t
by default as it will change many clocktables out there.  Let the
user decides whether she wants to turn this on.

lisp/org.el

@@ -2931,7 +2931,7 @@ See `org-time-clocksum-format' for more on time clock formats."
   :version "24.3"
   :type 'boolean)
 
-(defcustom org-time-clocksum-use-effort-durations t
+(defcustom org-time-clocksum-use-effort-durations nil
   "When non-nil, \\[org-clock-display] uses effort durations.
 E.g. by default, one day is considered to be a 8 hours effort,
 so a task that has been clocked for 16 hours will be displayed