Btrfs: remove unused code when creating and merging reloc trees

In commit 5bc7247a (Btrfs: fix broken nocow after balance) we started
abusing the rtransid and otransid fields of root items from relocation
trees to fix some issues with nodatacow mode. However later in commit
ba8b0289

 (Btrfs: do not reset last_snapshot after relocation) we
dropped the code that made use of those fields but did not remove
the code that sets those fields.

So just remove them to avoid confusion.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Josef Bacik <jbacik@fb.com>