Useless return statement in validate_sort_dir

In watcher/api/controllers/v1/utils.py, in
the validate_sort_dir method has a return statement,
however the return value is exactly the parameter's value.

This is misleading, so I removed it.

Change-Id: I18c5c7853a5afedac88431347712a4348c9fd5dd
Closes-Bug: #1546917

watcher/api/controllers/v1/audit.py

@@ -263,7 +263,7 @@ class AuditsController(rest.RestController):
                                sort_key, sort_dir, expand=False,
                                resource_url=None, audit_template=None):
         limit = api_utils.validate_limit(limit)
-        sort_dir = api_utils.validate_sort_dir(sort_dir)
+        api_utils.validate_sort_dir(sort_dir)
 
         marker_obj = None
         if marker: