Remove extraneous OTM codes and their species duplicates

We want to add a uniqueness constraint on `Species` objects:
    `(instance, common_name, genus, species, cultivar, other_part_of_name)` should be unique.
But we have duplicates in our own data -- 6 cases where two OTM codes have the same species info.
For those 6 pairs we'll keep one and kill the other.
Luckily, no production instance has a tree or i-Tree code override pointing to one of the species we want to kill.