Make the url file: handler be less clever

Make the url file: handler be less clever

* doc/misc/url.texi (file/ftp): Remove mention of the
url-directory-index-file variable, which is no longer consulted.

* lisp/url/url-file.el (url-file-build-filename): Remove the DWIM
code from the file: handler (bug#30195): It would look for
index.html in a directory if we asked it to fetch the directory.
Determining what to do in a directory should be left up to the
programs that use this low-level library.  If the library decides
to load a different file than we specified, then things start
falling apart, as demonstrated by this bug report.