rm onesignal-gradle-plugin so substitution works

The onesignal-gradle-plugin currently breaks the Gradle
`dependencySubstitution` feature we introduced in the last commit so we
removed it from the example project.

Adding`googleServices { disableVersionCheck = true }`
was required since the `onesignal-gradle-plugin` auto disables this
part of the `com.google.gms.google-services` for us. The current
OneSignal-Android-SDK doesn't work with it since it uses version ranges
which is a re-existing issue.

Author: jkasten2

Examples/OneSignalDemo/app/build.gradle

@@ -1,4 +1,3 @@
-apply plugin: 'com.onesignal.androidsdk.onesignal-gradle-plugin'
 apply plugin: 'com.android.application'
 
 android {
@@ -46,6 +45,7 @@ android {
     task flavorSelection() {
         if (getGradle().getStartParameter().getTaskRequests().toString().contains("Gms")) {
             apply plugin: 'com.google.gms.google-services'
+            googleServices { disableVersionCheck = true }
         } else {
             apply plugin: 'com.huawei.agconnect'
         }