Minor klib cleanups

Review: https://jetbrains.team/p/kt/reviews/8401

- Code style
- Drop unused properties in kotlin-native/build.gradle
- kotlin-native/build.gradle: drop unused import
- use final instead of open when possible

This is just a cleanup commit. It neither fixes any issue/bug, nor
introduces new behaviour.

Feel free to revert this commit if you find out that it breaks something

kotlin-native/gradle/kotlinGradlePlugin.gradle

@@ -1,5 +1,3 @@
-import org.jetbrains.kotlin.UtilsKt
-
 def kotlinRoot = ext.properties.containsKey("rootBuildDirectory") ? file("${ext.rootBuildDirectory}/..") : rootProject.projectDir
 File versionPropertiesFile = new File(kotlinRoot, "gradle/versions.properties")
 def versionProperties = new Properties()