[Datasets] Revert "Enable streaming executor by default (ray-project#…

…32493)" (ray-project#33485)

The reverted commit broke some RL tests.

![image](https://user-images.githubusercontent.com/26107013/226484663-c30f3466-80cf-4e98-84dc-b6250cf60a13.png)

python/ray/data/context.py

@@ -76,7 +76,7 @@
 # Whether to use the streaming executor. This only has an effect if the new execution
 # backend is enabled.
 DEFAULT_USE_STREAMING_EXECUTOR = bool(
-    int(os.environ.get("RAY_DATASET_USE_STREAMING_EXECUTOR", "1"))
+    int(os.environ.get("RAY_DATASET_USE_STREAMING_EXECUTOR", "0"))
 )
 
 # Whether to eagerly free memory (new backend only).