Revert "Modified to provide backward compatibility."

This reverts commit 01b2ca0a.

src/zc/buildout/testing.py

@@ -364,11 +364,7 @@ def buildoutSetUp(test):
     def create_signature_file_list(path,
                                    private_key_file='private.pem',
                                    certificate_file='public.pem'):
-        try:
-          from slapos.signature import createPrivateKeyAndCertificateFile
-        except ImportError:
-          return private_key_file, certificate_file
-
+        from slapos.signature import createPrivateKeyAndCertificateFile
         private_key_file = os.path.join(path, private_key_file)
         certificate_file = os.path.join(path, certificate_file)
         createPrivateKeyAndCertificateFile(certificate_file, private_key_file)