Remove license statement from notebooks

[renuka010]

This PR removes license statement from the following notebooks in examples.

adversarial_training.ipynb
cifar10_resnet.ipynb
flax_example.ipynb
haiku_example.ipynb
lookahead_mnist.ipynb
mlp_mnist.ipynb
differentially_private_sgd.ipynb
reduce_on_plateau.ipynb

Addressing issue #755

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