Replace format_price for Budget#formatted_amount

ApplicationHelper#format_price and Budget#formatted_amount has the same
objective and code, but the Budget#formatted_amount method also uses the
currency of the Budget to correctly give currencies format.

By replacing usage of format_price with formatted_amount we can remove
format_price and have a single location for currency format logic.

app/helpers/application_helper.rb

@@ -57,10 +57,6 @@ module ApplicationHelper
     SiteCustomization::ContentBlock.block_for(name, locale)
   end
 
-  def format_price(number)
-    number_to_currency(number, precision: 0, locale: I18n.locale)
-  end
-
   def kaminari_path(url)
     "#{root_url.chomp("\/")}#{url}"
   end