Reduce log severity of QWK-packet removal error log message to WARNING

This allows each call to ftp_remote() to specify the log level for any remove
failure. All others file-removal failures will continue to log at ERROR level.

I get this particular (harmless) error a lot, so just reducing to warning:
ftp  ... !ERROR 13 (Permission denied) (line 860) removing file: path/to/*.qwk