docs: def delete_variant(self, variant):
          """Delete observation in database
  
          This means that we take down the observations variable with one.
          If 'observations' == 1 we remove the variant. If variant was homozygote
          we decrease 'homozygote' with one.
          Also remove the family from array 'families'.
  
          Args:
              variant (dict): A variant dictionary
  
          """
          mongo_variant = self.get_variant(variant)
          
          if mongo_variant:
              
              if mongo_variant['observations'] == 1:
                  LOG.debug("Removing variant {0}".format(
                      mongo_variant.get('_id')
                  ))
                  message = self.db.variant.delete_one({'_id': variant['_id']})
              else:
                  LOG.debug("Decreasing observations for {0}".format(
                      mongo_variant.get('_id')
                  ))
                  message = self.db.variant.update_one({
                      '_id': mongo_variant['_id']
                      },{
                          '$inc': {
                              'observations': -1,
                              'homozygote': - (variant.get('homozygote', 0)),
                              'hemizygote': - (variant.get('hemizygote', 0)),
                          },
                          '$pull': {
                              'families': variant.get('case_id')
                          }
                      }, upsert=False)
          return