Identify rman files and delete them
If the database does not know about those files, you should catalog them in RMAN:

catalog start with '/path/to/files';


After that you can use your above commands for identifying or deleting these files.

Deleting old backups in a safe way is achieved by running DELETE OBSOLETE, with the required retention set in configuration or specified in the command (e.g. DELETE OBSOLETE RECOVERY WINDOW OF 14 DAYS;).