Major Description
This pull request removes the public developer parser API for external parsers. Given everything a parser depends on (e.g. logical plans and expressions) are internal and not stable, external parsers will break with every release of Spark. It is a bad idea to create the illusion that Spark actually supports pluggable parsers. In addition, this also reduces incentives for 3rd party projects to contribute parse improvements back to Spark.
The number of applications that are using this feature is small (as far as I know it came down from two to one as of Jan 2016, and will be 0 once we have better ansi SQL support).
