Revert "fix: migrate signBlob to iamcredentials.googleapis.com" (#563)

Reverts googleapis/google-auth-library-python#553

We have received reports that this is breaking users. See internal issue 161506225.
diff --git a/tests/test_iam.py b/tests/test_iam.py
index e20eeba..4367fe7 100644
--- a/tests/test_iam.py
+++ b/tests/test_iam.py
@@ -81,7 +81,7 @@
     def test_sign_bytes(self):
         signature = b"DEADBEEF"
         encoded_signature = base64.b64encode(signature).decode("utf-8")
-        request = make_request(http_client.OK, data={"signedBlob": encoded_signature})
+        request = make_request(http_client.OK, data={"signature": encoded_signature})
         credentials = make_credentials()
 
         signer = iam.Signer(request, credentials, mock.sentinel.service_account_email)