Removed double counting of expand when estimating cardinality

This may not look like much but it makes benchmarks green, at least locally...
I don't think we can have cardinality(exp) since it will lead to double counting when we build plans on top of expand, e.g. selection-expand will first count the cardinality of it lhs (i.e. of expand) and in the next iteration we add that contribution again. This will penalize long chains of expands.