Remove insecure custom profile background feature.

This feature is insecure by design. It allows any user to change anyone's custom background, and this allows for annoyance, or even a form of impersonation by making the target profile look different.
There's even a proof of concept exploit in the wild: http://jsfiddle.net/W56w5/3/embedded/result/
Besides security, this feature causes inconsistency and confusion as described in jshackles/Enhanced_Steam#67.