[Test] Deleted obsolete lines in test_custom_onnx_exec.py

tests/test_custom_onnx_exec.py

 import numpy as np
-# import onnx
-# from onnx import AttributeProto, GraphProto, TensorProto, helper
 from onnx import TensorProto, helper
 
 import finn.core.execute_custom_node as ex_cu_node