dbc.py: fix SQL syntax for  DELETE

Get rid of SQL errors: 
Traceback (most recent call last):
  File "/usr/local/lib/python3.4/dist-packages/itc_kit/gui/menus.py", line 377, in on_clear_all_clicked
    dbc.remove_all_notifications()
  File "/usr/local/lib/python3.4/dist-packages/itc_kit/db/dbc.py", line 125, in remove_all_notifications
    db.execute("DELETE * FROM Notification")
sqlite3.OperationalError: near "*": syntax error