remove redundant dependency (#785)

sbt-scala-module brings in sbt-header itself

Author: SethTisue

project/plugins.sbt

@@ -3,4 +3,3 @@ addSbtPlugin("org.portable-scala" % "sbt-scalajs-crossproject" % "1.3.2")
 addSbtPlugin("org.portable-scala" % "sbt-scala-native-crossproject" % "1.3.2")
 addSbtPlugin("org.scala-js" % "sbt-scalajs" % "1.20.1")
 addSbtPlugin("org.scala-native" % "sbt-scala-native" % "0.5.9")
-addSbtPlugin("de.heikoseeberger" % "sbt-header" % "5.10.0")