Revert "Expose session start file in __file__. (#1078)"

This reverts commit 1185d12.

Author: Carreau

ipykernel/ipkernel.py

@@ -3,7 +3,6 @@
 import asyncio
 import builtins
 import getpass
-import os
 import signal
 import sys
 import threading
@@ -127,11 +126,6 @@ def __init__(self, **kwargs):
             compiler_class=XCachingCompiler,
         )
         self.shell.displayhook.session = self.session
-
-        jupyter_session_name = os.environ.get('JPY_SESSION_NAME')
-        if jupyter_session_name:
-            self.shell.user_ns['__file__'] = jupyter_session_name
-
         self.shell.displayhook.pub_socket = self.iopub_socket
         self.shell.displayhook.topic = self._topic("execute_result")
         self.shell.display_pub.session = self.session