[FEA] Remove redundant parameter in strings::detail::create_chars_child_column

[ttnghia]

Currently, strings::detail::create_chars_child_column has the following signature:

std::unique_ptr<column> create_chars_child_column(
  size_type strings_count,
  size_type bytes,
  rmm::cuda_stream_view stream        = rmm::cuda_stream_default,
  rmm::mr::device_memory_resource* mr = rmm::mr::get_current_device_resource());

and its implementation is simply as

std::unique_ptr<column> create_chars_child_column(cudf::size_type strings_count,
                                                  cudf::size_type total_bytes,
                                                  rmm::cuda_stream_view stream,
                                                  rmm::mr::device_memory_resource* mr)
{
  return make_numeric_column(
    data_type{type_id::INT8}, total_bytes, mask_state::UNALLOCATED, stream, mr);
}

Thus, the first parameter is redundant. We need to refactor this to remove that parameter. This is a simple task. However, this API is used in many places thus after refactoring this function we still need to spend some time to fix its usage.