Remove buffer field in PerstArtifact

[arcanefoam]

The buffer field is private with no access methods, thus it is only used internally. However, AFAICT it is only accessed in the setData and getData methods. In the setData, it is used under the assumption that the ArtifactData is serializabe. There might be cases in which the data being loaded into ECCO is not, e.g. EMF EObjects are not (by default).
In the getData, it is used to recreate the data from the buffer in case the data is null., but since the buffer can not be filled/set elsewhere there is no way this is actually helpfull.

[arcanefoam]

The buffer is part of a serialization mechanism. So no change needed