Clean up code which checked presence of os.{stat,lstat,chmod} (#11643)

diff --git a/Lib/dbm/dumb.py b/Lib/dbm/dumb.py
index 6cef72a..864ad37 100644
--- a/Lib/dbm/dumb.py
+++ b/Lib/dbm/dumb.py
@@ -278,8 +278,7 @@
     __del__ = close
 
     def _chmod(self, file):
-        if hasattr(self._os, 'chmod'):
-            self._os.chmod(file, self._mode)
+        self._os.chmod(file, self._mode)
 
     def __enter__(self):
         return self