Remove an unused method

Author: lfkellogg

firebase-appdistribution/src/main/java/com/google/firebase/appdistribution/internal/InstallActivity.java

@@ -169,9 +169,4 @@ private void startAndroidPackageInstallerIntent() {
     LogWrapper.getInstance().v("Kicking off install as new activity");
     startActivity(intent);
   }
-
-  @VisibleForTesting
-  FirebaseAppDistribution getFirebaseAppDistributionInstance() {
-    return FirebaseAppDistribution.getInstance();
-  }
 }