Remove extraneous Flush ?

[bartelink]

Based on https://github.com/confluentinc/confluent-kafka-dotnet/issues/1449#issuecomment-723680820 it seems to me that the Flush in ProduceAsync can be removed from this impl https://github.com/jet/FsKafka/blob/master/src/FsKafka/FsKafka.fs#L210

More research needed - this is working fine, but maybe the perf can be improved by this...

mhowlett, if you see this, would appreciate any insight you can offer ;)