Add dropOp mutation to example

[vmrajas]

Currently in index.js, there is no example of dropping all data, while keeping the schema.

It was found out that the format of dropOp query is unintuitive. Hence, adding an example to index.js

Related Discuss link: https://discuss.dgraph.io/t/how-do-i-drop-data-using-the-js-library/9744

---

This change is 

[CLAassistant]

All committers have signed the CLA.

[vardhanapoorv]

A minor comment.

Reviewed 2 of 2 files at r1.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @vmrajas)

---

examples/simple/index.js, line 6 at r1 (raw file):

// Create a client stub.
function newClientStub() {
    return new dgraph.DgraphClientStub("localhost:9080", grpc.credentials.createInsecure());

If we don't pass, we would do that anyway. That's the reason to remove it right? Or it's something else?