remove "_cum" flux variables

[teixeirak]

We have 3 flux variables measured over time increments other than 1yr:

NEE_cum_C
GPP_cum_C
R_eco_cum_C

There are just 34 records total, and they don't seem useful for much, but add confusion. Non-urgently, I'd like to delete those records (along with variable definitions and any plots, sites, methods, etc. used only in association with those).