Remove explicit use of IOLoop.clear_current()

IOLoop.clear_current is deprecated since Tornado 6.2
If proactive manipulation of the running loop is required, the API docs
say to use asyncio.set_event_loop()

Author: dwskoog

streamz/utils_test.py

@@ -43,13 +43,11 @@ def double(x):
 
 @contextmanager
 def pristine_loop():
-    IOLoop.clear_current()
     loop = IOLoop()
     try:
         yield loop
     finally:
         loop.close(all_fds=True)
-        IOLoop.clear_current()
 
 
 def gen_test(timeout=10):