Assume make-directory handler follows new API

Assume make-directory handler follows new API

Suggested by Michael Albinus (Bug#58919#56).
* lisp/files.el (files--ensure-directory): Omit recently-added arg
MKDIR, since it is now always make-directory again.  All uses
changed.
(make-directory): Assume the make-directory handler follows the
new API where it yields non-nil if DIR already exists.  This
reverts some of the recent changes in this area, and simplifies
this funciton.