Unused and badly defined ktau in biogeochem

[Whyborn]

In trying to introduce the datetime_module to the serial driver, I found an uninitialised variable was being passed to biogeochem in CASAONLY_LUC, see here. As far as I can tell, the ktau being passed to that routine call is uninitialised. I checked the actual biogeochem routine, and it seems that variable isn't being used at all in that routine, so should be removed.