Don't assert when storing a second deactivated sensor in the cache




This removes an assert that checks whether a deactivated
sensor is stored another time. This happens when you check,
modify, uncheck a sensor for one preset, then go to another
preset of the same paintop type that has that sensor activated
and you deactivate it.

The problem seems to be, afaict, that we try to remember
the settings of deactivated sensors, but do that per paintop
instance, not per preset.

There are two proper solutions:

* remove the caching of custom settings for deactivated sensors
* save customized but deactivated sensors to the presets

The question is, which one shall it be?