Revert "Use truststore in appliance_manager.py instead"

This reverts commit a2628042e8.

gns3server/controller/appliance_manager.py

@@ -37,14 +37,6 @@ import logging
 log = logging.getLogger(__name__)
 
 
-try:
-    import truststore
-    truststore.inject_into_ssl()
-    log.info("Using system certificate store for SSL connections")
-except ImportError:
-    pass
-
-
 class ApplianceManager:
     """
     Manages appliances