Independent of "Toggle Source Preview" being ON or OFF, an not wanted/needed Source Layer can not be deleted. The complete Focus Merge process must be done again .
May be I am interpreting the trash bin in a wrong way, but what else should it be for...?
I also tried to drag and drop the unwanted Focus Merge layer (the blue, selected one) into the trash bin, but this didn't help either...
Greetings,
Roland