Remove get_password_hash, unused

spwd is removed in Python 3.13. But fortunately, this function itself is
never used.

Part of: #1073

examples/the_basics.py

@@ -10,7 +10,6 @@ from __future__ import print_function
 import hashlib
 import io
 import os
-import spwd
 
 import mitogen.core
 import mitogen.master
@@ -57,21 +56,6 @@ def streamy_download_file(context, path):
     }
 
 
-def get_password_hash(username):
-    """
-    Fetch a user's password hash.
-    """
-    try:
-        h = spwd.getspnam(username)
-    except KeyError:
-        return None
-
-    # mitogen.core.Secret() is a Unicode subclass with a repr() that hides the
-    # secret data. This keeps secret stuff out of logs. Like blobs, secrets can
-    # also be serialized.
-    return mitogen.core.Secret(h)
-
-
 def md5sum(path):
     """
     Return the MD5 checksum for a file.