ext4: remove unneeded forward declaration

Remove unneeded forward declaration of ext4_destroy_lazyinit_thread().

Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/20241218145414.1422946-4-shikemeng@huaweicloud.com


Signed-off-by: Theodore Ts'o <tytso@mit.edu>