Remove lastmod timestamps from the sitemap

[pmac]

The <lastmod> data in our sitemap files is gathered and stored in the www-sitemap-generator repo by an involved process that involves spinning up a bedrock instance and hitting every URL on the site using etags to determine if a page has been modified, and then updating a data file with the results. After discussion with our SEO expert we've decided that we no longer need the <lastmod> data, which will allow us to greatly reduce the complexity of the sitemap generation process. We should do the following:

• Remove the use of etags and lastmod data from bedrock and its sitemap files
• Remove the generation of the etag and lastmod data in the www-sitemap-generator repo

All else should be able to remain the same for now.