Cleanup: remove generic_tracer_vertdiff_M and g_tracer_vertdiff_M?

[andrew-c-ross]

I don't see any uses of these two routines in the current generic tracer code, and I think they're a vestige from the CMIP5 era? As far as I can see all code is now using generic_tracer_vertidff_G, which we could also rename to just generic_tracer_vertdiff.