Remove useless line

Author: japgolly

library/tests/src/test/scala/japgolly/scalajs/react/core/ReusabilityTest.scala

@@ -145,7 +145,6 @@ object ReusabilityTest extends TestSuite {
   private class DerivesByRef {
     class X
     implicit val x: Reusability[X] = Reusability.never
-    val _ = x
     case class Y(x: X)
     implicit val y: Reusability[Y] = Reusability.derive[Y]
     val y1 = Y(new X)