Remove mc6 fval and fval_unit columns

[madison-feshuk]

fval and fval_unit are deprecated columns in mc6

select * from mc6 where fval is not NULL; returns 0 rows

Make schema change to delete fval and fval_unit from mc6 table. Once complete, review mc6_mthds.R to remove mention of these columns. Currently, they're always created by default, but are not populated. Test processing some endpoints for issues.