Clean up exception syntax

python/google/protobuf/descriptor_pool.py

@@ -57,8 +57,6 @@ directly instead of this class.
 
 __author__ = 'matthewtoia@google.com (Matt Toia)'
 
-import sys
-
 from google.protobuf import descriptor
 from google.protobuf import descriptor_database
 from google.protobuf import text_encoding