## What changes were proposed in this pull request?
1. Rename DataFrame.scala Dataset.scala, since the class is now named Dataset.
2. Remove LegacyFunctions. It was introduced in Spark 1.6 for backward compatibility, and can be removed in Spark 2.0.

## How was this patch tested?
Should be covered by existing unit/integration tests.

Author: Reynold Xin <rxin@databricks.com>

Closes #11704 from rxin/SPARK-13880.