Remove Soccer .nn files (#2615)

UnitySDK/Assets/ML-Agents/Examples/Soccer/Brains/GoalieLearning.asset

     vectorActionDescriptions:
     - 
     vectorActionSpaceType: 0
-  model: {fileID: 11400000, guid: b6dd703e0bf914268a4e110ad85ab7a9, type: 3}
+  model: {fileID: 0}
   inferenceDevice: 0

UnitySDK/Assets/ML-Agents/Examples/Soccer/Brains/StrikerLearning.asset

     vectorActionDescriptions:
     - 
     vectorActionSpaceType: 0
-  model: {fileID: 11400000, guid: 0df960ed78a964cd4a671395390ac522, type: 3}
+  model: {fileID: 0}
   inferenceDevice: 0

UnitySDK/Assets/ML-Agents/Examples/Soccer/TFModels.meta

-fileFormatVersion: 2
-guid: 26481c347fb204ec4a732603c8954a01
-folderAsset: yes
-timeCreated: 1512788689
-licenseType: Free
-DefaultImporter:
-  externalObjects: {}
-  userData: 
-  assetBundleName: 
-  assetBundleVariant: