Remove unused budget attributes

Tags and help links can be edited, but aren't used anywhere. Since we
don't know what the intended behavior was, I'm removing them for now.

My best guess is tags were supposed to be used so investments for a
budget can only be assigned tags present in the budget. Achieving that
behavior wouldn't be a trivial task.

app/controllers/admin/budgets_controller.rb

@@ -70,9 +70,6 @@ class Admin::BudgetsController < Admin::BaseController
       descriptions = Budget::Phase::PHASE_KINDS.map { |p| "description_#{p}" }.map(&:to_sym)
       valid_attributes = [:phase,
                           :currency_symbol,
-                          :help_link,
-                          :milestone_tag_list,
-                          :valuation_tag_list,
                           administrator_ids: [],
                           valuator_ids: [],
                           tracker_ids: []