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/utils.py

@@ -47,7 +47,6 @@ def validate_sort_dir(sort_dir):
         raise wsme.exc.ClientSideError(_("Invalid sort direction: %s. "
                                          "Acceptable values are "
                                          "'asc' or 'desc'") % sort_dir)
-    return sort_dir
 
 
 def validate_search_filters(filters, allowed_fields):