gh-150882: Remove unused shutil._ensure_directory

Introduced in #8295 alongside shutil.unpack_archive() as a helper that
created the parent directory of each extracted member.

Orphaned by gh-146581 (GH-146591), the ZIP path-traversal fix, which
reworked _unpack_zipfile() to create directories inline and deleted the
last _ensure_directory(targetpath) call site in the same change.

Since then the private helper has had no caller: a word-boundary search
across Lib, Modules, Python, Objects and Include finds zero references
outside its own definition, and a GitHub code search finds no downstream
importers.

Author: gaborbernat

Lib/shutil.py

@@ -1307,12 +1307,6 @@ def unregister_unpack_format(name):
     """Removes the pack format from the registry."""
     del _UNPACK_FORMATS[name]
 
-def _ensure_directory(path):
-    """Ensure that the parent directory of `path` exists"""
-    dirname = os.path.dirname(path)
-    if not os.path.isdir(dirname):
-        os.makedirs(dirname)
-
 def _unpack_zipfile(filename, extract_dir):
     """Unpack zip `filename` to `extract_dir`
     """