Remove a conditional branch related to Python2

Author: caolonghao

python/paddle/fluid/distributed/ps_pb2.py

@@ -17,7 +17,7 @@
 
 import sys
 
-_b = sys.version_info[0] < 3 and (lambda x: x) or (lambda x: x.encode('latin1'))
+_b = (lambda x: x.encode('latin1'))
 from google.protobuf.internal import enum_type_wrapper
 from google.protobuf import descriptor as _descriptor
 from google.protobuf import message as _message