openflow_input: change gentable delete error case

It's easier for the controller if deleting a nonexistent entry is not an error.
openflow_input: add bsn_flow_checksum
loci: support bsn_flow_checksum
loci: remove dead code
Merge into master from pull request #193:
bsn_flow_checksum extension (https://github.com/floodlight/loxigen/pull/193)