Disable debug flag on CSS fetches: fixes server-side caching

We got some complaints about a flood today of API fetches for
style updates from iOS app, which it turns out are marked as
uncacheable due to the 'debug=true' parameter.

Removed this, which also means the CSS files get minified.

Change-Id: I0b49369d26b80e6b096afaea7156ca69b408a47c