Fix: invalid syntax with python 3.5 in test_ust.py

Fix: invalid syntax with python 3.5 in test_ust.py

The following error was encountered while running the test suite with
python 3.5 :

  File "./tools/context/test_ust.py", line 99
    client: lttngtest.Controller = lttngtest.LTTngClient(test_env, log=tap.diagnostic)
          ^
  SyntaxError: invalid syntax

Remove the type hint.

Change-Id: I2d8c39d092b8a1659208b89aaca1cf11ca53866a
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>