btrfs: remove remaing full_sync logic from btrfs_sync_file

The logic to check if the inode is already in the log can now be
simplified since we always wait for the ordered extents to complete
before deciding whether the inode needs to be logged. The big comment
about it can go away too.

CC: Filipe Manana <fdmanana@suse.com>
Suggested-by: Filipe Manana <fdmanana@suse.com>
[ code and changelog copied from mail discussion ]
Signed-off-by: David Sterba <dsterba@suse.com>