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.

polkadot/tests/purge_chain_works.rs

@@ -57,7 +57,6 @@ async fn purge_chain_rocksdb_works() {
 		assert!(cmd.wait().unwrap().success());
 		assert!(tmpdir.path().join("chains/rococo_dev").exists());
 		assert!(tmpdir.path().join("chains/rococo_dev/db/full").exists());
-		assert!(tmpdir.path().join("chains/rococo_dev/db/full/parachains").exists());
 
 		// Purge chain
 		let status = Command::new(cargo_bin("polkadot"))
@@ -102,7 +101,6 @@ async fn purge_chain_paritydb_works() {
 		assert!(cmd.wait().unwrap().success());
 		assert!(tmpdir.path().join("chains/rococo_dev").exists());
 		assert!(tmpdir.path().join("chains/rococo_dev/paritydb/full").exists());
-		assert!(tmpdir.path().join("chains/rococo_dev/paritydb/parachains").exists());
 
 		// Purge chain
 		let status = Command::new(cargo_bin("polkadot"))
@@ -118,8 +116,6 @@ async fn purge_chain_paritydb_works() {
 		// Make sure that the chain folder exists, but `db/full` is deleted.
 		assert!(tmpdir.path().join("chains/rococo_dev").exists());
 		assert!(!tmpdir.path().join("chains/rococo_dev/paritydb/full").exists());
-		// Parachains removal requires calling "purge-chain --parachains".
-		assert!(tmpdir.path().join("chains/rococo_dev/paritydb/parachains").exists());
 	})
 	.await;
 }

substrate/client/cli/src/commands/purge_chain_cmd.rs

@@ -48,7 +48,7 @@ pub struct PurgeChainCmd {
 impl PurgeChainCmd {
 	/// Run the purge command
 	pub fn run(&self, database_config: DatabaseSource) -> error::Result<()> {
-		let db_path = database_config.path().ok_or_else(|| {
+		let db_path = database_config.path().and_then(|p| p.parent()).ok_or_else(|| {
 			error::Error::Input("Cannot purge custom database implementation".into())
 		})?;