Remove references to MongoDB

We don't support it any more.

Context:

https://gruntwork.zendesk.com/agent/tickets/108564

https://github.com/gruntwork-io/terraform-aws-mongodb

```
As of November, 2019, we are putting this repo on hold and will not be investing in further improvements to it. 
```

I was going to just submit a PR, but the Docker quick start is broken (I'll submit a different issue for that)