btrfs: remove stale prototype of btrfs_write_inode

This function no longer exists, was removed in 3c427693 ("Btrfs: fix
btrfs_write_inode vs delayed iput deadlock").

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>