[Test] remove generated .onnx files

tests/brevitas/test_brevitas_act_export.py

+import os
 import numpy as np
 import torch
 import brevitas.onnx as bo
@@ -41,3 +42,4 @@ def test_brevitas_act_export(abits, narrow_range, max_val):
     inp_tensor = torch.from_numpy(inp_tensor).float()
     expected = b_act.forward(inp_tensor).detach().numpy()
     assert np.isclose(produced, expected, atol=1e-3).all()
+    os.remove(export_onnx_path)

tests/transformation/test_topk_insert.py

+import os
 import onnx
 from finn.util.test import get_test_model_trained
 import brevitas.onnx as bo
@@ -56,3 +57,4 @@ def test_topk_insert(k):
     output_pysim_topk = output_pysim_topk.astype(np.int).flatten()
 
     assert np.array_equal(output_golden_topk, output_pysim_topk)
+    os.remove(export_onnx_path)