Fix issue when creating session with non-default window name

Before this change if I executed a code block like the following

  #+begin_src tmux :session default:foo
    echo hello world
  #+end_src

and no session named "default" exists, then first a session with a
window named "foo" would be created by (ob-tmux--create-session) and
then (sometimes) an additional window with the name "foo" would be
created by (ob-tmux--create-window).  This has lead in my case to some
follow up failures.  Therefore this change where always a window with
the default name is created for new sessions.

ob-tmux.el

@@ -227,7 +227,7 @@ Argument OB-SESSION: the current ob-tmux session."
      "-d" ;; just create the session, don't attach.
      "-c" (expand-file-name "~") ;; start in home directory
      "-s" (ob-tmux--session ob-session)
-     "-n" (ob-tmux--window-default ob-session))))
+     "-n" org-babel-tmux-default-window-name)))
 
 (defun ob-tmux--create-window (ob-session)
   "Create a tmux window in session if it does not yet exist.