[SPARK-52714][SDP] Remove unused comment arg in append_flow decorator

### What changes were proposed in this pull request?

In Spark Declarative Pipelines (SDP), users can define append flows in Python using the [append_flow](https://github.com/apache/spark/blob/e3321aa44ea255365222c491657b709ef41dc460/python/pyspark/pipelines/api.py#L34-L41) decorator. The append_flow decorator currently accepts a `comment` arg. However, there is no way for user to see flow comments as of now. Therefore, this argument is unused and not referenced in function body.

```py
def append_flow(
    *,
    target: str,
    name: Optional[str] = None,
    comment: Optional[str] = None,                 # <--- Removing
    spark_conf: Optional[Dict[str, str]] = None,
    once: bool = False,
) -> Callable[[QueryFunction], None]:
```

This PR removes the field.

### Why are the changes needed?

The `comment` arg is not being used anywhere and having it in the API will confuse the user thinking they can see flow comments somewhere.

### Does this PR introduce _any_ user-facing change?

Yes, the previously optional `comment` arg is removed from the `append_flow` API. However, SDP has not been released yet (pending release in v4.1), so no user should be impacted by this change.

### How was this patch tested?

Examined all testcases to make sure none of the current append_flow usage is supplying this argument

### Was this patch authored or co-authored using generative AI tooling?

No

Closes #51404 from JiaqiWang18/SPARK-52714-remove-append_flow-comment.

Authored-by: Jacky Wang <jacky.wang@databricks.com>
Signed-off-by: Sandy Ryza <sandy.ryza@databricks.com>

Author: jackywang-db

python/pyspark/pipelines/api.py

@@ -35,7 +35,6 @@ def append_flow(
     *,
     target: str,
     name: Optional[str] = None,
-    comment: Optional[str] = None,
     spark_conf: Optional[Dict[str, str]] = None,
     once: bool = False,
 ) -> Callable[[QueryFunction], None]:
@@ -44,7 +43,6 @@ def append_flow(
 
     :param name: The name of the flow. If unspecified, the query function's name will be used.
     :param target: The name of the dataset this flow writes to. Must be specified.
-    :param comment: Description of the flow. If unspecified, the dataset's comment will be used.
     :param spark_conf: A dict whose keys are the conf names and values are the conf values. \
         These confs will be set when the flow is executed; they can override confs set for the \
         destination, for the pipeline, or on the cluster.