Remove dependency on sbt-spark-package

[zouzias]

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Additional context

• Is this plugin deprecated? databricks/sbt-spark-package#48
• Ported to sbt 1.0 databricks/sbt-spark-package#41

[yeikel]

What is the alternative?

[zouzias]

There is no need to use this plugin any more, since it does not provide any extra functionality. sbt-spark-package gives a bit more easy-handling in the setup of sbt but it could be removed safely.