Docfix: using `org-sort' with a double prefix doesn't delete duplicates.

Thanks to Paweł Menich for spotting this.

doc/org.texi

@@ -1443,8 +1443,7 @@ creation time, scheduled time, deadline time), by priority, by TODO keyword
 (in the sequence the keywords have been defined in the setup) or by the value
 of a property.  Reverse sorting is possible as well.  You can also supply
 your own function to extract the sorting key.  With a @kbd{C-u} prefix,
-sorting will be case-sensitive.  With two @kbd{C-u C-u} prefixes, duplicate
-entries will also be removed.
+sorting will be case-sensitive.
 @orgcmd{C-x n s,org-narrow-to-subtree}
 Narrow buffer to current subtree.
 @orgcmd{C-x n b,org-narrow-to-block}

lisp/org.el

@@ -7960,8 +7960,7 @@ and still retain the repeater to cover future instances of the task."
 
 (defun org-sort (with-case)
   "Call `org-sort-entries', `org-table-sort-lines' or `org-sort-list'.
-Optional argument WITH-CASE means sort case-sensitively.
-With a double prefix argument, also remove duplicate entries."
+Optional argument WITH-CASE means sort case-sensitively."
   (interactive "P")
   (cond
    ((org-at-table-p) (org-call-with-arg 'org-table-sort-lines with-case))