python: Remove unused imports.

Found by pychecker.

python/ovs/db/data.py

@@ -12,12 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-import errno
-import logging
-import os
 import re
-import select
-import sys
 import uuid
 
 import ovs.poller

python/ovs/stream.py

@@ -17,7 +17,6 @@ import logging
 import os
 import select
 import socket
-import sys
 
 import ovs.poller
 import ovs.socket_util