* progmodes/python.el: Simplified imenu implementation.

* progmodes/python.el: Simplified imenu implementation.
(python-nav-jump-to-defun): Remove command.
(python-mode-map): Use `imenu' instead.
(python-nav-list-defun-positions-cache)
(python-imenu-include-defun-type, python-imenu-make-tree)
(python-imenu-subtree-root-label, python-imenu-index-alist):
Remove vars.
(python-nav-list-defun-positions, python-nav-read-defun)
(python-imenu-tree-assoc, python-imenu-make-element-tree)
(python-imenu-make-tree, python-imenu-create-index): Remove
functions.
(python-mode): Update to interact with imenu by setting
`imenu-extract-index-name-function' only.