remove multibackend (#3555)

* remove multibackend

* oops

* goodbye pointless tests
diff --git a/tests/conftest.py b/tests/conftest.py
index 1952ec1..8605f3a 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -6,7 +6,6 @@
 
 import pytest
 
-from cryptography.hazmat.backends import _available_backends
 from cryptography.hazmat.backends.openssl import backend as openssl_backend
 
 from .utils import check_backend_support, select_backends, skip_if_empty
@@ -19,7 +18,7 @@
 def pytest_generate_tests(metafunc):
     if "backend" in metafunc.fixturenames:
         names = metafunc.config.getoption("--backend")
-        selected_backends = select_backends(names, _available_backends())
+        selected_backends = select_backends(names, [openssl_backend])
 
         filtered_backends = []
         required = metafunc.function.requires_backend_interface