Remove "XXX/XXX" default sp_auth_credentials: related to MKSP-125.

Remove "XXX/XXX" default sp_auth_credentials: related to MKSP-125.

According to the very helpful X-MK-Component header,
http://sp-mkws.indexdata.com/service-proxy/ is now using Service-Proxy
0.39. That means that the fix to MKWS-125 is in production (as it was
fixed in v0.38), so I can get rid of the horrible hack where MKWS has
been deliberately sending the credentials XXX/XXX then the application
provides none, in the hope that they're incorrect. Hurrah!