code: def del_bg(self, key):
          
          if key not in VALID_BG_KEYS:
              raise ValueError("Invalid bg key: {}".format(key))
          if key in self.h5["bg_data"]:
              del self.h5["bg_data"][key]
          else:
              msg = "No bg data to clear for '{}' in {}.".format(key, self)
              warnings.warn(msg)
docs: Remove the background image data
  
          Parameters
          ----------
          key: str
              One of :const:`VALID_BG_KEYS`