Remove unused arg from stop_standing_subprocess. (#518)

diff --git a/mobly/utils.py b/mobly/utils.py
index b4f8536..a9f065c 100644
--- a/mobly/utils.py
+++ b/mobly/utils.py
@@ -316,7 +316,7 @@
     return proc
 
 
-def stop_standing_subprocess(proc, kill_signal=signal.SIGTERM):
+def stop_standing_subprocess(proc):
     """Stops a subprocess started by start_standing_subprocess.
 
     Before killing the process, we check if the process is running, if it has