btrfs: remove more obsolete v0 extent ref declarations

The extent references v0 have been superseded long time go, there are
some unused declarations of access helpers. We can safely remove them
now. The struct btrfs_extent_ref_v0 is not used anywhere, but struct
btrfs_extent_item_v0 is still part of a backward compatibility check in
relocation.c and thus not removed.

Signed-off-by: David Sterba <dsterba@suse.com>