The `flatten` function used `concat` and `slice` which created a lot of intermediary
object allocations. Because `flatten` is used from query any benchmark which
used query would exhibit high minor GC counts.
PR Close#30468
This commit removes code duplication where we had 2 versions of a
`flatten` utility. Moreover this change results in queries using
a non-recursive version of `flatten` which should result in a better
performance of query refresh operations.
PR Close#29547