Revert "fix: check client is not nil before closing"

This reverts commit 94285cd94d.

qemu/qemu-kernel.go

@@ -408,13 +408,7 @@ func (q System) Command(user, cmd string) (output string, err error) {
 		flog.Debug().Err(err).Msg("ssh connection")
 		return
 	}
-	defer func() {
-		if client != nil {
-			client.Close()
-		} else {
-			log.Debug().Msg("why client is nil?")
-		}
-	}()
+	defer client.Close()
 
 	session, err := client.NewSession()
 	if err != nil {