Remove filecache.el

Remove filecache.el

This library doesn't work well in recent Emacsen, and more modern
workflows generally supersede it.

* doc/emacs/emacs.texi (Top)
* doc/emacs/files.texi (Files): Update menu.
(File Name Cache): Remove section.
* doc/lispref/minibuf.texi (Text from Minibuffer): Remove comment.
* doc/misc/tramp.texi (Frequently Asked Questions): Don't mention it.
* lisp/bindings.el (minibuffer-local-map): Unbind 'C-TAB'.
* lisp/filecache.el: Delete file.
* test/lisp/help-tests.el (help-tests--test-keymap)
(help-tests-substitute-command-keys/keymaps): Adjust.