Remove obsolete API, public key update

I ran the test and got an error:
```
=== RUN TestUpdatePublicKey
--- FAIL: TestUpdatePublicKey (0.73 seconds)
        users_test.go:184: Could not update key *******: Bad HTTP status; wanted 200 got 405
```

I couldn't find when the API became obsolete, but anyway [API documentation](https://developer.github.com/v3/users/keys/) says,

> Public keys are immutable. If you need to update a public key, 
> remove the key and create a new one instead.

So I removed the obsolete API to solve test failure.