chore: remove package-lock.json (#1237)

I just noticed that we have both a package-lock.json and a yarn.lock on
this branch.

- yarn.lock was added in 176cfd4 (Jan 2017) and only updated once, in
  c59af29 (also in Jan 2017).
- package-lock.json was added in e636edf (Oct 2017) and updated on a
  number of occasions, the latest being in 2cb99ed (April 2019).

Let's standardize on Yarn, to match what we're doing in the "master"
branch and in other projects.

In order to get a working build I had to trash the yarn.lock and
regenerate it.