[SPARK-18104][DOC] Don't build KafkaSource doc
[SPARK-18104][DOC] Don't build KafkaSource doc

## What changes were proposed in this pull request?

Don't need to build doc for KafkaSource because the user should use the data source APIs to use KafkaSource. All KafkaSource APIs are internal.

## How was this patch tested?

Verified manually.

Author: Shixiong Zhu <b59cd5ce8cb5ae44d26fc6984be1f84cf74771a7@databricks.com>

Closes #15630 from zsxwing/kafka-unidoc.

(cherry picked from commit 7d10631c16b980adf1f55378c128436310daed65)
Signed-off-by: Shixiong Zhu <b59cd5ce8cb5ae44d26fc6984be1f84cf74771a7@databricks.com>

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