Revert "Add default loader for latest pyyaml (#4974)" (#5432)

This reverts commit d49e61e0.

docker/pulsar/scripts/gen-yml-from-env.py

@@ -46,7 +46,7 @@ if len(sys.argv) < 2:
 conf_files = sys.argv[1:]
 
 for conf_filename in conf_files:
-    conf = yaml.load(open(conf_filename), Loader=yaml.FullLoader)
+    conf = yaml.load(open(conf_filename))
 
     # update the config
     modified = False