`h2po4g` is unused in approx pCO2 solver

[briochemc]

I noticed that h2po4g is unused (thank you VScode + linter!):

darwin3/pkg/dic/carbon_chem.F

Line 425 in 24885b7

h2po4g = (pt*k1plocal*hguess*hguess) / stuff

I guess this line can be safely removed then. Not sure this is of any use, but I thought I'd point it out in case it is! :)

FWIW I stumbled on this while trying to translate this pCO2 solver from Follows et al., to MATLAB 😭 (to get away from Newton-solver issues in CO2sys.m).