Remove yarn lockfile

npm@5 has a `package-lock.json` which servers a similar purpose.

I initially wanted to keep both but Heroku won't deploy with both:

```
-----> Node.js app detected
-----> Build failed
 !     Two different lock files found: package-lock.json and yarn.lock
       Both npm and yarn have created lock files for this application,
       but only one can be used to install dependencies. Installing
       dependencies using the wrong package manager can result in
missing
       packages or subtle bugs in production.
       - To use npm to install your application's dependencies please
         delete
         the yarn.lock file.
         $ git rm yarn.lock
       - To use yarn to install your application's dependences please
         delete
         the package-lock.json file.
         $ git rm package-lock.json

       https://kb.heroku.com/why-is-my-node-js-build-failing-because-of-conflicting-lock-files
 !     Push rejected, failed to compile Node.js app.
 !     Push failed
```