`chain-spec`: `polkadot_staging_testnet` config removed (#7509)

* chain-spec: polkadot_staging_testnet config removed

The config has empty initial authority test. It is unusable.

```
$ polkadot export-blocks --chain polkadot-staging --from=0 --to=0
...
Thread 'main' panicked at 'Empty validator set for session 0 in genesis block!',
/home/miszka/.cargo/git/checkouts/substrate-7e08433d4c370a21/c446786/frame/session/src/lib.rs:463
```

* warning fix