Revert "NFS: add nfs_sb_deactive_async to avoid deadlock"

This reverts commit 324d003b0c.

The deadlock turned out to be caused by a workqueue limitation that has
now been worked around in the RPC code (see comment in rpc_free_task).

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/internal.h

@@ -329,7 +329,6 @@ extern int __init register_nfs_fs(void);
 extern void __exit unregister_nfs_fs(void);
 extern void nfs_sb_active(struct super_block *sb);
 extern void nfs_sb_deactive(struct super_block *sb);
-extern void nfs_sb_deactive_async(struct super_block *sb);
 
 /* namespace.c */
 #define NFS_PATH_CANONICAL 1