Remove unused attributes (#6631)

Author: alex

src/cryptography/hazmat/primitives/hashes.py

@@ -75,8 +75,6 @@ def __init__(
             raise TypeError("Expected instance of hashes.HashAlgorithm.")
         self._algorithm = algorithm
 
-        self._backend = backend
-
         if ctx is None:
             from cryptography.hazmat.backends.openssl.backend import (
                 backend as ossl,