Remove MXNet examples #1043

Job	Run time
Publish Image (earlystopping-medianstop, cmd/earlystopping/medianstop/v1beta1/Dockerfile) / Publish Image	4m 4s
Publish Image (suggestion-darts, cmd/suggestion/nas/darts/v1beta1/Dockerfile) / Publish Image	1m 30s
Publish Image (suggestion-enas, cmd/suggestion/nas/enas/v1beta1/Dockerfile) / Publish Image	5m 13s
Publish Image (suggestion-goptuna, cmd/suggestion/goptuna/v1beta1/Dockerfile) / Publish Image	6m 21s
Publish Image (suggestion-hyperband, cmd/suggestion/hyperband/v1beta1/Dockerfile) / Publish Image	4m 59s
Publish Image (suggestion-hyperopt, cmd/suggestion/hyperopt/v1beta1/Dockerfile) / Publish Image	2m 53s
Publish Image (suggestion-optuna, cmd/suggestion/optuna/v1beta1/Dockerfile) / Publish Image	1m 55s
Publish Image (suggestion-pbt, cmd/suggestion/pbt/v1beta1/Dockerfile) / Publish Image	1m 23s
Publish Image (suggestion-skopt, cmd/suggestion/skopt/v1beta1/Dockerfile) / Publish Image	2m 38s
	30m 56s

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