Fix sessionmanager test

IPython/html/services/sessions/sessionmanager.py

@@ -178,8 +178,5 b' class SessionManager(LoggingConfigurable):'
         """Deletes the row in the session database with given session_id"""
         # Check that session exists before deleting
         model = self.get_session(id=session_id)
-        if model is None:
-            raise TraitError("The session does not exist: %s" %session_id)
-        else: 
             self.cursor.execute("DELETE FROM session WHERE id=?", (session_id,))
             self.connection.commit()
  No newline at end of file

IPython/html/services/sessions/tests/test_sessionmanager.py

@@ -1,11 +1,9 b''
 """Tests for the session manager."""
 
-import os
-
 from unittest import TestCase
-from tempfile import NamedTemporaryFile
 
-from IPython.utils.tempdir import TemporaryDirectory
+from tornado import web
+
 from IPython.utils.traitlets import TraitError
 
 from ..sessionmanager import SessionManager
@@ -82,5 +80,5 b' class TestSessionManager(TestCase):'
         sm = SessionManager()
         session_id = sm.get_session_id()
         sm.save_session(session_id=session_id, name='test.ipynb', path='/path/to/', kernel_id='5678', ws_url='ws_url')
-        self.assertRaises(TraitError, sm.delete_session, session_id='23424') # Bad keyword
+        self.assertRaises(web.HTTPError, sm.delete_session, session_id='23424') # Bad keyword