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

Removes the knob image knob04.png. This knob corresponded to
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.

include/Knob.h

@@ -38,7 +38,7 @@ class TextFloat;
 
 enum knobTypes
 {
-	knobDark_28, knobBright_26, knobSmall_17, knobGreen_17, knobVintage_32, knobStyled
+	knobDark_28, knobBright_26, knobSmall_17, knobVintage_32, knobStyled
 } ;