Simplify generated SBT code - after ScalaFX 18.0.1-R28 JavaFX dependencies not needed

[jpsacha]

With ScalaFX 18.0.1-R28 and SBT 1.6 and newer there is no longer need to explicitly specify JavaFX dependencies and native dependencies. This significantly simplifies the SBT build file.

scalafx/scalafx#380