s390/hypfs: Remove obsoleted declaration for hypfs_dbfs_exit

The hypfs_dbfs_exit() have been removed since
commit 3325b4d8 ("s390/hypfs: factor out filesystem code"),
and now it is useless, so remove it.
Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Acked-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/hypfs/hypfs.h

@@ -78,7 +78,6 @@ struct hypfs_dbfs_file {
 	struct dentry		*dentry;
 };
 
-extern void hypfs_dbfs_exit(void);
 extern void hypfs_dbfs_create_file(struct hypfs_dbfs_file *df);
 extern void hypfs_dbfs_remove_file(struct hypfs_dbfs_file *df);