Remove useless param for create_groupingsets_path

Dear Postgres,

Recently I work on grouping sets and I find the last param numGroups of create_groupingsets_path is not used.
In create_groupingsets_path we use rollup->numGroups to do cost_agg. So I remove the param numGroups for
create_groupingsets_path.

I generate a diff.patch, which is sent as an attachment.
I really hope this can be committed to Postgres. Thank you a lot!

Best wishes to you!
zxuejing