Removes an unused knob and enum (knob04.png aka knobGreen_17)

[michaelgregorius]

Removes the knob image knob04.png. This knob corresponded to the enum value
knobGreen_17 which was not used anywhere in the code. To be able to
remove the enum value it was necessary to change the knob loading code
in Knob::onKnobNumUpdated. However, the changed implementation is more
explicit and therefore likely better to understand.

[michaelgregorius]

With the more explicit mapping of enums to file names in Knob::onKnobNumUpdated we should also consider to rename the knob images to something more explicit, e.g. name the images like the enums.

[Wallacoloo]

👍 looks fine to me.