Remove unused investments filter

We weren't using the "Feasible" filter anywhere in the
application, and it's hard for users to know the difference between
"Feasible" and "Not unfeasible".

app/controllers/concerns/investment_filters.rb

@@ -3,7 +3,7 @@ module InvestmentFilters
 
   class_methods do
     def investment_filters
-      %w[not_unfeasible feasible unfeasible unselected selected winners]
+      %w[not_unfeasible unfeasible unselected selected winners]
     end
   end