(insert-directory): Remove `ls-lisp` advice

(insert-directory): Remove `ls-lisp` advice

Rather than have `ls-lisp` advise `insert-directory`, make
`insert-directory` call `ls-lisp.el` code directly when needed.

* lisp/files.el (files--use-insert-directory-program-p): New function.
(insert-directory): Use it to delegate to `ls-lisp--insert-directory`
when applicable.

* lisp/ls-lisp.el (ls-lisp--insert-directory): Remove `orig-fun` arg.
Don't test `ls-lisp-use-insert-directory-program` or check for a magic
file name handler; it is now the caller's responsibility.
(insert-directory): Don't add advice any more.

* lisp/dired.el (ls-lisp-use-insert-directory-program): Don't declare it.
(dired-insert-directory): Use `files--use-insert-directory-program-p` instead.
(dired-use-ls-dired): Adjust docstring to refer to
`insert-directory-program` rather than "ls".