Remove two config variables pointing to blob storage container names

[simon-20]

As a developer,
I want to avoid having old config variables in the code which aren't strictly needed.

The app was originally designed to put XML files and individual ZIP files of those XML files into different Azure blob storage containers.

However, doing this meant we would have had to serve the XML and ZIP files from a different subdomain to the main Bulk Data Service website.

Thus, now everything is put into the $web blob storage container. So we no longer need two configuration variables providing the option of different containers. Reference to these should be removed, and replaced with a single config variable.