Change default of org-koma-letter-signature to nil.

* ox-koma-letter.el(org-koma-letter-signature): set to nil.

The previoues value corresponds to the default value cf. the the koma
manual pp. 183.

Signed-off-by: Alan Schmitt <alan.schmitt@polytechnique.org>

contrib/lisp/ox-koma-letter.el

@@ -130,7 +130,7 @@ function may be given.  Functions must return a string."
   :group 'org-export-koma-letter
   :type 'string)
 
-(defcustom org-koma-letter-signature "\\usekomavar{fromname}"
+(defcustom org-koma-letter-signature nil
   "String used as the signature."
   :group 'org-export-koma-letter
   :type 'string)