[SPARK-12855][MINOR][SQL][DOC][TEST] remove spark.sql.dialect from doc and test

## What changes were proposed in this pull request?

Since developer API of plug-able parser has been removed in #10801 , docs should be updated accordingly.

## How was this patch tested?

This patch will not affect the real code path.

Author: Daoyuan Wang <daoyuan.wang@intel.com>

Closes #11758 from adrian-wang/spark12855.