Fix purge-db edge case (#2747)

## Issue Addressed

Currently, if you launch the beacon node with the `--purge-db` flag and the `beacon` directory exists, but one (or both) of the `chain_db` or `freezer-db` directories are missing, it will error unnecessarily with: 
```
Failed to remove chain_db: No such file or directory (os error 2)
```

This is an edge case which can occur in cases of manual intervention (a user deleted the directory) or if you had previously run with the `--purge-db` flag and Lighthouse errored before it could initialize the db directories.

## Proposed Changes

Check if the `chain_db`/`freezer_db` exists before attempting to remove them. This prevents unnecessary errors.

beacon_node/src/config.rs

@@ -36,17 +36,21 @@ pub fn get_config<E: EthSpec>(
     // If necessary, remove any existing database and configuration
     if client_config.data_dir.exists() && cli_args.is_present("purge-db") {
         // Remove the chain_db.
-        fs::remove_dir_all(client_config.get_db_path().ok_or("Failed to get db_path")?)
+        let chain_db = client_config.get_db_path().ok_or("Failed to get db_path")?;
+        if chain_db.exists() {
+            fs::remove_dir_all(chain_db)
                 .map_err(|err| format!("Failed to remove chain_db: {}", err))?;
+        }
 
         // Remove the freezer db.
-        fs::remove_dir_all(
-            client_config
+        let freezer_db = client_config
             .get_freezer_db_path()
-                .ok_or("Failed to get freezer db path")?,
-        )
+            .ok_or("Failed to get freezer db path")?;
+        if freezer_db.exists() {
+            fs::remove_dir_all(freezer_db)
                 .map_err(|err| format!("Failed to remove chain_db: {}", err))?;
         }
+    }
 
     // Create `datadir` and any non-existing parent directories.
     fs::create_dir_all(&client_config.data_dir)