NFS: switch back to using kill_anon_super

NFS switch to open coding kill_anon_super in 7b14a213
("nfs: don't call bdi_unregister") to avoid the extra bdi_unregister
call.  At that point bdi_destroy was called in nfs_free_server and
thus it required a later freeing of the anon dev_t.  But since
0db10944 ("nfs: Convert to separately allocated bdi") the bdi has
been free implicitly by the sb destruction, so this isn't needed
anymore.

By not open coding kill_anon_super, nfs now inherits the fix in
dc3216b1 ("super: ensure valid info"), and we remove the only
open coded version of kill_anon_super.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Message-Id: <20230831052940.256193-1-hch@lst.de>
Signed-off-by: Christian Brauner <brauner@kernel.org>