💚 Fix test cleanup for self-signed certs support

Some tests were leading to the REQUESTS_CA_BUNDLE being set without
it being set upfront by the developer, and it was never being cleared.

Running the api_client tests after that were then affected by this
envvar.

src/openforms/tests/test_self_signed_certificates.py

@@ -50,6 +50,8 @@ def tearDownClass(cls):
 
         if cls._original_requests_ca is not None:
             os.environ["REQUESTS_CA_BUNDLE"] = cls._original_requests_ca
+        elif "REQUESTS_CA_BUNDLE" in os.environ:
+            del os.environ["REQUESTS_CA_BUNDLE"]
 
 
 class SelfSignedCertificateTests(RestoreOriginalCABundleMixin, TestCase):