Remove opt_filter optional parameter in GroupsAccumulator:merge

[alamb]

          That is why I return error for now, we can remove the `opt_filter` after couple of months

Originally posted by @jayzhan211 in #13463 (comment)

The idea is that the parameter is never used, so we can remove it to simplify GroupsAccumulator implementations.

@jayzhan211s PRs above throws an error if this assumption is not true.

If we haven't hit any errors in a few months, we should just remove the parameter entirely