Removed different treatment of cluster size calculations for EtaCorrection method.
The cluster size has no relation to the centroid method and should be calculated in the same way for all.

If using filter_cluster=True the results before and after this change are the same, as only matched clusters remain.
However, with filter_cluster=False the old code will count every cluster (even non-matched) for the EtaCorrection method.
I think this is the wrong way to do it.