update dbus-loader: Remove Loader from safe_load()

As I commented on the commit c0bf88e, c0bf88ea05
python3-validity.service does not start, at least on Debian buster, since
safe_load() does not have arg Loader on its prototype.
Removing Loader here,
config = yaml.safe_load(configfd)
makes it back to normal starting.

dbus_service/dbus-service

@@ -241,7 +241,7 @@ def main():
     # Load and perform basic validation of config file.
     try:
         with (args.configpath / 'dbus-service.yaml').open(mode='rt') as configfd:
-            config = yaml.safe_load(configfd, Loader=yaml.BaseLoader)
+            config = yaml.safe_load(configfd)
     except FileNotFoundError:
         # No configuration file. Create default
         config = {'user_to_sid': {}}