Do not generate CSS source maps in production

[TNL-271](https://openedx.atlassian.net/browse/TNL-271) eliminate broken links to .map files

These are for development environment only, and we did not distribute them to production servers. So this change only removes reference to never-existing files, making debug console output a bit cleaner in production.

@talbs @cahrens @andy-armstrong @polesye @olmar