Move shell-dir-cookie-re feature into Dirtrack mode.

Move shell-dir-cookie-re feature into Dirtrack mode.

* lisp/dirtrack.el (dirtrack-list): Eliminate unused third element.
(dirtrack): Merge code for handling relative filenames in prompt
from shell-dir-cookie-watcher.
(dirtrack-debug-message): New arg to avoid excess format calls.

* lisp/shell.el (shell-dir-cookie-re): Variable deleted.
(shell-dir-cookie-watcher): Function deleted.
(shell-mode): Don't use shell-dir-cookie-re, since it is redundant
with dirtrack-mode.