Remove MXNet examples #458

Job	Run time
e2e (v1.25.12, long-running-resume,from-volume-resume,median-stop)	14m 8s
e2e (v1.25.12, grid,bayesian-optimization,tpe,multivariate-tpe,cma-es,hyperband)	19m 19s
e2e (v1.25.12, file-metrics-collector,pytorchjob-mnist)	12m 13s
e2e (v1.25.12, median-stop-with-json-format,file-metrics-collector-with-json-format)	12m 2s
e2e (v1.26.6, long-running-resume,from-volume-resume,median-stop)	14m 11s
e2e (v1.26.6, grid,bayesian-optimization,tpe,multivariate-tpe,cma-es,hyperband)	17m 10s
e2e (v1.26.6, file-metrics-collector,pytorchjob-mnist)	12m 54s
e2e (v1.26.6, median-stop-with-json-format,file-metrics-collector-with-json-format)	11m 45s
e2e (v1.27.3, long-running-resume,from-volume-resume,median-stop)	14m 42s
e2e (v1.27.3, grid,bayesian-optimization,tpe,multivariate-tpe,cma-es,hyperband)	16m 9s
e2e (v1.27.3, file-metrics-collector,pytorchjob-mnist)	12m 39s
e2e (v1.27.3, median-stop-with-json-format,file-metrics-collector-with-json-format)	12m 27s
	2h 49m 39s

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