Remove two unused imports of JavaConverters.

This removes the last references to `JavaConverters` in our
codebase, excluding scalalib overrides and comments.

Author: sjrd

js-envs/src/main/scala/org/scalajs/jsenv/ExternalJSEnv.scala

@@ -19,7 +19,6 @@ import org.scalajs.core.tools.jsdep.ResolvedJSDependency
 import java.io.{ Console => _, _ }
 import scala.io.Source
 
-import scala.collection.JavaConverters._
 import scala.concurrent.{Future, Promise}
 import scala.util.Try
 

js-envs/src/main/scala/org/scalajs/jsenv/nodejs/AbstractNodeJSEnv.scala

@@ -24,7 +24,6 @@ import org.scalajs.core.tools.logging.NullLogger
 import org.scalajs.jsenv._
 import org.scalajs.jsenv.Utils.OptDeadline
 
-import scala.collection.JavaConverters._
 import scala.concurrent.TimeoutException
 import scala.concurrent.duration._