Remove an unused method (#3367)

Author: lfkellogg

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

@@ -20,7 +20,6 @@
 import android.os.Build;
 import android.os.Bundle;
 import android.provider.Settings;
-import androidx.annotation.VisibleForTesting;
 import androidx.appcompat.app.AppCompatActivity;
 import androidx.core.content.FileProvider;
 import com.google.firebase.appdistribution.FirebaseAppDistribution;
@@ -169,9 +168,4 @@ private void startAndroidPackageInstallerIntent() {
     LogWrapper.getInstance().v("Kicking off install as new activity");
     startActivity(intent);
   }
-
-  @VisibleForTesting
-  FirebaseAppDistribution getFirebaseAppDistributionInstance() {
-    return FirebaseAppDistribution.getInstance();
-  }
 }