[SPARK-16155][DOC] remove package grouping in Java docs
[SPARK-16155][DOC] remove package grouping in Java docs

## What changes were proposed in this pull request?

In 1.4 and earlier releases, we have package grouping in the generated Java API docs. See http://spark.apache.org/docs/1.4.0/api/java/index.html. However, this disappeared in 1.5.0: http://spark.apache.org/docs/1.5.0/api/java/index.html.

Rather than fixing it, I'd suggest removing grouping. Because it might take some time to fix and it is a manual process to update the grouping in `SparkBuild.scala`. I didn't find anyone complaining about missing groups since 1.5.0 on Google.

Manually checked the generated Java API docs and confirmed that they are the same as in master.

Author: Xiangrui Meng <9616cf314f60cbadfd3d5d51110130ad7923defb@databricks.com>

Closes #13856 from mengxr/SPARK-16155.

(cherry picked from commit 857ecff1d8268b28bb287e47cda370c87afe9d41)
Signed-off-by: Xiangrui Meng <9616cf314f60cbadfd3d5d51110130ad7923defb@databricks.com>

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