Fix dataframe operation after read (#370)

* Fix dataframe operation after read

Author: dewan-c

src/sagemaker_xgboost_container/distributed_gpu/dask_data_utils.py

@@ -32,7 +32,7 @@ def _read_data(local_path: str, content_type: str) -> (DataFrame, Series):
 
     target_column = dataframe.columns[0]
     labels = dataframe[target_column]
-    features = dataframe[dataframe.columns.difference([target_column])]
+    features = dataframe[dataframe.columns[1:]]
 
     return features, labels