Remove columnar.chunk_group.row_count column

We can already infer that from `columnar.stripe.row_count`.

Note that this would require having an additional column in `columnar.stripe` for the last `chunk_group` in given stripe (e.g: `columnar.stripe.last_chunk_group_row_count`).

I made something for that in below branch, which doesn't care about last chunk group row count tough.
https://github.com/citusdata/citus/compare/col/remove-cgroup-row-cnt