integration/ct: drop unused function (#426)
integration/ct: drop unused function (#426)

There is now a ct.DigitallySigned.String(), but this function is
actually no longer used.
RJPercival/trillian,daviddrysdale/trillian,AlCutter/trillian,codingllama/trillian,RJPercival/trillian,gdbelvin/trillian,google/trillian,daviddrysdale/trillian,google/trillian,Martin2112/trillian,gdbelvin/trillian,gtank/trillian,Martin2112/trillian,codingllama/trillian,Martin2112/trillian,gdbelvin/trillian,gtank/trillian,AlCutter/trillian,daviddrysdale/trillian