erofs: drop unnecessary WARN_ON() in erofs_kill_sb()

Previously, .kill_sb() will be called only after fill_super fails.
It will be changed [1].

Besides, checking for s_magic in erofs_kill_sb() is unnecessary from
any point of view.  Let's get rid of it now.

[1] https://lore.kernel.org/r/20230731-flugbereit-wohnlage-78acdf95ab7e@brauner

Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Christian Brauner <brauner@kernel.org>
Link: https://lore.kernel.org/r/20230801014737.28614-1-hsiangkao@linux.alibaba.com