Removing duplicate example

Removing the `rfid_dump_block0` example as it is not significantly different from the `rfid_default_keys` example. Both dump block 0, after authentication using a (default) Key A. In the case of `rfid_dump_block0` it is just using the FF FF FF FF FF FF = factory default key, while `rfid_default_keys` also tries some other known keys if that one fails.