Remove MICADO_img_cal_background template

MW suggested we remove MICADO_img_cal_background:

Just one question: Is 'MICADO_img_cal_background' really necessary? Sky (background) exposures are a natural part of the science templates. And since you expect all acquisitions to be applicable here, I guess you want this template to be part of any science OB.

which is fine.

So I'll add it as another 'virtual' template.