integration: remove done TODO
integration: remove done TODO

No longer any cgo dependence from the Merkle tree code.

codingllama/trillian,Martin2112/trillian,gtank/trillian,AlCutter/trillian,gtank/trillian,RJPercival/trillian,Martin2112/trillian,Martin2112/trillian,gdbelvin/trillian,daviddrysdale/trillian,google/trillian,codingllama/trillian,gdbelvin/trillian,google/trillian,AlCutter/trillian,gdbelvin/trillian,daviddrysdale/trillian,RJPercival/trillian,daviddrysdale/trillian