Remove unnecessary import

Author: Jonathan Sparling

onnx_tf/handlers/backend/softmax_cross_entropy_loss.py

@@ -1,6 +1,5 @@
 import tensorflow as tf
 
-from onnx_tf.common import exception
 from onnx_tf.handlers.backend_handler import BackendHandler
 from onnx_tf.handlers.handler import onnx_op
 from onnx_tf.handlers.handler import tf_func