Remove warnings.simplefilter to suppress ResourceWarning

This commit removes warnings.simplefilter from
tempest.lib.services.clients code to suppress annoying ResourceWarning
like following.

 ResourceWarning: unclosed file <_io.FileIO....

Change-Id: I9e96cfc062bbdaf73f8a4efb9af9dc581c96994f
diff --git a/tempest/lib/services/clients.py b/tempest/lib/services/clients.py
index e7ac423..90debd9 100644
--- a/tempest/lib/services/clients.py
+++ b/tempest/lib/services/clients.py
@@ -18,7 +18,6 @@
 import importlib
 import inspect
 import sys
-import warnings
 
 from debtcollector import removals
 from oslo_log import log as logging
@@ -35,7 +34,6 @@
 from tempest.lib.services import placement
 from tempest.lib.services import volume
 
-warnings.simplefilter("once")
 LOG = logging.getLogger(__name__)