CCBUG:375720 Disable the gradient map filter for filter layers

This doesn't work currently because the gradient is stored in a
problematic way in the filter config:

With an absolute path, which makes the file non-portable

Could not find gradient "C:/Users/GC/AppData/Roaming/krita/gradients/kC5644.svg"

With a plain name

Could not find gradient "skin1"

Or apparently without any name, I guess for the autogenerated gradients

Could not find gradient "C:/Users/GC/AppData/Roaming/krita/gradients\\.svg"

To fix this properly, we will need to sit down and fix the resource system.