[Fix] Removed the metafile for a non existing folder (#464)

unity-environment/Assets/ML-Agents/Examples/Basic/Materials.meta

-fileFormatVersion: 2
-guid: 0f9b2a7b3f61045b8a791eeae8175dc5
-folderAsset: yes
-timeCreated: 1506189694
-licenseType: Pro
-DefaultImporter:
-  userData: 
-  assetBundleName: 
-  assetBundleVariant: