Hide group name only on budgets with one group

In the form of creating a new investment was hiding the name of the
group if it had only one heading, but could be confusing to users if
there are, for example, five different groups of one heading.

The solution:

- If the budget has one group and one heading, the heading selector is
  hidden.
- If the budget has one group and more than one heading, the group name
  is hidden.
- If the budget has more than one group, the group name appears
  regardless of the number of headings.

app/models/budget/group.rb

@@ -30,10 +30,6 @@ class Budget
       all.sort_by(&:name)
     end
 
-    def single_heading_group?
-      headings.count == 1
-    end
-
     private
 
       def generate_slug?