apacheGH-41623: [Docs] Remove the warning for arrow::dataset (apach…

…e#43148)

### Rationale for this change

According to the discussion in [41623](apache#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: apache#41623

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

docs/source/cpp/dataset.rst

@@ -27,11 +27,6 @@ Tabular Datasets
 .. seealso::
    :doc:`Dataset API reference <api/dataset>`
 
-.. warning::
-
-    The ``arrow::dataset`` namespace is experimental, and a stable API
-    is not yet guaranteed.
-
 The Arrow Datasets library provides functionality to efficiently work with
 tabular, potentially larger than memory, and multi-file datasets. This includes: