Remove offest tracking in Kafka plugins

[Dieterbe]

We don't use it anymore

[woodsaj]

the offset tracking is only needed when the "offset" for the kafka-mdm-in plugin or kafka-cluster plugin are set to "last".

This config setting never makes sense. Data is internally buffered until a "chunk" is filled up, so data will be lost if MT doesn't replay old data at startup (offset=oldest, or offset=<time duration>) ensuring all un-written chunks are restored in their entirety.

If the user can tolerate the lost data, due to having multiple replicas, then they are better off just using "offset=newest"

So, lets just get rid of "offset=last" support and get rid of the offset tracking.