fix: remove checks for ancient versions of Cryptography (#596)

Refs #595 (comment) 

I see no point in checking whether someone is running a version of https://github.com/pyca/cryptography/ from 2014 that doesn't even compile against modern versions of OpenSSL anymore.

google/auth/crypt/_cryptography_rsa.py

@@ -25,23 +25,10 @@
 from cryptography.hazmat.primitives import serialization
 from cryptography.hazmat.primitives.asymmetric import padding
 import cryptography.x509
-import pkg_resources
 
 from google.auth import _helpers
 from google.auth.crypt import base
 
-_IMPORT_ERROR_MSG = (
-    "cryptography>=1.4.0 is required to use cryptography-based RSA " "implementation."
-)
-
-try:  # pragma: NO COVER
-    release = pkg_resources.get_distribution("cryptography").parsed_version
-    if release < pkg_resources.parse_version("1.4.0"):
-        raise ImportError(_IMPORT_ERROR_MSG)
-except pkg_resources.DistributionNotFound:  # pragma: NO COVER
-    raise ImportError(_IMPORT_ERROR_MSG)
-
-
 _CERTIFICATE_MARKER = b"-----BEGIN CERTIFICATE-----"
 _BACKEND = backends.default_backend()
 _PADDING = padding.PKCS1v15()

google/auth/crypt/es256.py

@@ -25,22 +25,10 @@
 from cryptography.hazmat.primitives.asymmetric.utils import decode_dss_signature
 from cryptography.hazmat.primitives.asymmetric.utils import encode_dss_signature
 import cryptography.x509
-import pkg_resources
 
 from google.auth import _helpers
 from google.auth.crypt import base
 
-_IMPORT_ERROR_MSG = (
-    "cryptography>=1.4.0 is required to use cryptography-based ECDSA " "algorithms"
-)
-
-try:  # pragma: NO COVER
-    release = pkg_resources.get_distribution("cryptography").parsed_version
-    if release < pkg_resources.parse_version("1.4.0"):
-        raise ImportError(_IMPORT_ERROR_MSG)
-except pkg_resources.DistributionNotFound:  # pragma: NO COVER
-    raise ImportError(_IMPORT_ERROR_MSG)
-
 
 _CERTIFICATE_MARKER = b"-----BEGIN CERTIFICATE-----"
 _BACKEND = backends.default_backend()