Make dired-default-directory obsolete.

Make dired-default-directory obsolete.

As far as I can tell, it was never used outside of dired-smart-shell-command.
It doesn't seem worth keeping around as a separate function.

* lisp/dired-x.el (dired-default-directory-alist, dired-default-directory):
Mark as obsolete.
(dired-smart-shell-command): Just call dired-current-directory.

* doc/misc/dired-x.texi (Multiple Dired Directories): Remove mentions
of dired-default-directory-alist and dired-default-directory.
Move dired-smart-shell-command here...
(Miscellaneous Commands): ... from here.