Delete script directory (#937)

99% certain we don't use this file;

- [script](https://github.com/just-the-docs/just-the-docs/search?q=script) has no non-`<script>`/doc uses
- [build](https://github.com/just-the-docs/just-the-docs/search?q=build) has no mentions
- we don't need to manually build / package the gem anymore