Remove explicit IOLoop.make_current in utils_test

Tornado 4.2 add an optional keyword to IOLoop(make_current=False)
Tornado 6.3 changes that to make_current=True so the default behavior is
to install the loop as the current thread's loop.

Author: dwskoog

streamz/utils_test.py

@@ -45,7 +45,6 @@ def double(x):
 def pristine_loop():
     IOLoop.clear_current()
     loop = IOLoop()
-    loop.make_current()
     try:
         yield loop
     finally: