Remove stray raise in library code

Added in galaxyproject#15340 by accident.
Fixes:

```
RuntimeError: No active exception to reraise
  File "galaxy/web/framework/decorators.py", line 337, in decorator
    rval = func(self, trans, *args, **kwargs)
  File "galaxy/webapps/galaxy/api/library_contents.py", line 130, in index
    for content in traverse(library.root_folder):
  File "galaxy/webapps/galaxy/api/library_contents.py", line 84, in traverse
    can_access, folder_ids = trans.app.security_agent.check_folder_contents(
  File "galaxy/model/security.py", line 1510, in check_folder_contents
    raise
```
in https://sentry.galaxyproject.org/share/issue/38df4490ff61415090c319ad0e16f1fa/

lib/galaxy/model/security.py

@@ -1507,7 +1507,6 @@ def check_folder_contents(self, user, roles, folder, hidden_folder_ids=""):
             return True, ""
         action = self.permitted_actions.DATASET_ACCESS
 
-        raise
         lddas = (
             self.sa_session.query(self.model.LibraryDatasetDatasetAssociation)
             .join("library_dataset")