public boolean isRemovable() {
    return isRemovable;
  }
If aggregate function is removable, {@linkplain AggregateFunction#remove} should be
implemented.

@param removable whether the aggregate function is removable
iotdb-api/udf-api/src/main/java/org/apache/iotdb/udf/api/customizer/config/AggregateFunctionConfig.java