virtio: console: remove unnecessary null test before debugfs_remove_recursive

Fix checkpatch warning:
WARNING: debugfs_remove_recursive(NULL) is safe this check is probably not required

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: virtualization@lists.linux-foundation.org
Reviewed-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>