arrow

GH-41623: [Docs] Remove the warning for `arrow::dataset` (#43148)



### Rationale for this change

According to the discussion in [41623](https://github.com/apache/arrow/issues/41623), the warning message indicating that the `arrow::dataset`  is experimental is no longer necessary.

### What changes are included in this PR?

Remove warning message

### Are these changes tested?

Yes. I have verified the changes by building the documentation.

### Are there any user-facing changes?

Yes. the updated documentation will be visible to users.

* GitHub Issue: #41623

Authored-by: Hyunseok Seo <hsseo0501@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>