Remove kotlin extension plugin

* Plugin has been deprecated, and is not needed for kotlin support from the SDK

Author: Jeasmine

OneSignalSDK/onesignal/build.gradle

@@ -6,7 +6,6 @@ ext {
 
 apply plugin: 'com.android.library'
 apply plugin: 'kotlin-android'
-apply plugin: 'kotlin-android-extensions'
 
 android {
     compileSdkVersion rootProject.buildVersions.compileSdkVersion