// 
db.collection("CollectionName").document("Documentname")
                        .delete()
                        .addOnSuccessListener(new OnSuccessListener<Void>() {
                            @Override
                            public void onSuccess(Void aVoid) {
                                Log.d(TAG, "DocumentSnapshot successfully deleted!");
                            }
                        })
                        .addOnFailureListener(new OnFailureListener() {
                            @Override
                            public void onFailure(@NonNull Exception e) {
                                Log.w(TAG, "Error deleting document", e);
                            }
                        });