Remove investment filters in groups

The interface was a bit confusing, since after clicking on "See
unfeasible investments" (or similar), we were on a page where no
investments were shown.

Besides, since commit 7e3dd47d5, the group page is only linked from the
"my ballot" page, through a link inviting the user to vote in that
group, and it's only possible to vote selected investments (which is the
default filter during the final voting phase).

The only reason we had these links here was these links weren't present
in the investments page. But they're present there since commit
04605d5d5, so we don't need them in the group page anymore.

app/helpers/budgets_helper.rb

@@ -16,9 +16,7 @@ module BudgetsHelper
   end
 
   def css_for_ballot_heading(heading)
-    return "" if current_ballot.blank? || @current_filter == "unfeasible"
-
-    current_ballot.has_lines_in_heading?(heading) ? "is-active" : ""
+    current_ballot&.has_lines_in_heading?(heading) ? "is-active" : ""
   end
 
   def current_ballot