Remove ambiguous unit "cup"

The DFC doesn't actually specify which cup it means. I don't expect
anyone providing "cup" as unit to measure their produce and expect it to
calculate as a regular volume. It can just be seen as items.

app/services/weights_and_measures.rb

@@ -41,7 +41,6 @@ class WeightsAndMeasures
       1.0 => { 'name' => 'L', 'system' => 'metric' },
       1000.0 => { 'name' => 'kL', 'system' => 'metric' },
 
-      0.25 => { 'name' => 'cu', 'system' => 'imperial' },
       4.54609 => { 'name' => 'gal', 'system' => 'imperial' },
     }
   }.freeze

engines/dfc_provider/app/services/quantitative_value_builder.rb

@@ -80,10 +80,6 @@ class QuantitativeValueBuilder < DfcBuilder
       ["volume", nil, 0.01]
     when quantity_unit.DECILITRE
       ["volume", nil, 0.1]
-    when quantity_unit.CUP
-      # Interpreted as metric cup, not US legal cup.
-      # https://github.com/datafoodconsortium/taxonomies/issues/8
-      ["volume", nil, 0.25]
     when quantity_unit.GALLON
       ["volume", nil, 4.54609]