remove old nas test case (#1789)

test/config_test/examples/mnist-nas.test.yml

-authorName: nni
-experimentName: default_test
-maxExecDuration: 5m
-maxTrialNum: 4
-trialConcurrency: 2
-
-tuner:
-  codeDir: ../../../examples/tuners/random_nas_tuner
-  classFileName: random_nas_tuner.py
-  className: RandomNASTuner
-trial:
-  codeDir: ../../../examples/trials/mnist-nas/classic_mode/
-  command: python3 mnist.py --batch_num 100
-  gpuNum: 0
-  nasMode: classic_mode
-
-useAnnotation: true
-multiPhase: false
-multiThread: false
-
-trainingServicePlatform: local