[SPARK-13882][SQL] Remove org.apache.spark.sql.execution.local

## What changes were proposed in this pull request?
We introduced some local operators in org.apache.spark.sql.execution.local package but never fully wired the engine to actually use these. We still plan to implement a full local mode, but it's probably going to be fairly different from what the current iterator-based local mode would look like. Based on what we know right now, we might want a push-based columnar version of these operators.

Let's just remove them for now, and we can always re-introduced them in the future by looking at branch-1.6.

## How was this patch tested?
This is simply dead code removal.

Author: Reynold Xin <rxin@databricks.com>

Closes #11705 from rxin/SPARK-13882.