Remove everything related to USDA code because it doesn't match OTM code

We thought that OTM codes matched USDA codes except when there's a cultivar. But that's not the case. For example:
    Silver maple:  OTM code "ACSA1", USDA code "ACSA2"
    Sugar maple :  OTM code "ACSA2", USDA code "ACSA3"
    Willow acacia: OTM code "ACSA3", USDA code "ACSA10"

So it doesn't make sense to match USDA code against OTM code. I'm removing everything related to USDA code.