Delete full db directory with `purge-chain` subcommand (#1786)

Closes #1767 

Until now the `purge-chain` command would only remove the `full`
subfolder of the db folder. However there is also the `parachains` db
that currently remains and can cause problems on node restart.

Example wiht old code:
```
polkadot purge-chain --database paritydb --base-path /tmp/some-folder
Are you sure to remove "/tmp/some-folder/chains/polkadot/paritydb/full"? [y/N]: y
"/tmp/some-folder/chains/polkadot/paritydb/full" removed.
```
In this case `/tmp/some-folder/chains/polkadot/paritydb/parachains`
would remain and might cause problem on node restart because of version
conflicts as described in #1767. After this PR the whole
`/tmp/some-folder/chains/polkadot/paritydb` folder will be deleted.