Remove package-lock.json

From my understanding, a project should only have one lock file. At the moment, we have both `package-lock.json` and `yarn.lock`.

Because we're currently deploying using `yarn.lock` (on Vercel) and it seems to be more active/used, I've decided to drop the `package-lock.json`.

https://github.com/hackclub/site/commits/main/package-lock.json

https://github.com/hackclub/site/commits/main/yarn.lock