Remove IOLoop.clear_instance from utils_test.py

https://www.tornadoweb.org/en/stable/ioloop.html#tornado.ioloop.IOLoop.clear_instance
clear_instance is now an alias for clear_current and has been deprecated
since Tornado 5.0

Author: dwskoog

streamz/utils_test.py

@@ -43,15 +43,13 @@ def double(x):
 
 @contextmanager
 def pristine_loop():
-    IOLoop.clear_instance()
     IOLoop.clear_current()
     loop = IOLoop()
     loop.make_current()
     try:
         yield loop
     finally:
         loop.close(all_fds=True)
-        IOLoop.clear_instance()
         IOLoop.clear_current()