Use form_tag in management area investments search form

Since this form isn't associated to an object, using `form_tag` instead
of `form_for` simplifies the code.

app/components/management/budgets/investments/search_component.html.erb

@@ -1,4 +1,4 @@
-<%= form_for(Budget::Investment.new, url: url, as: :budget_investment, method: :get) do |f| %>
+<%= form_tag(url, method: :get) do |f| %>
   <div class="row">
     <div class="small-12 medium-6 column">
       <%= text_field_tag :search, "" %>
@@ -19,7 +19,7 @@
 
   <div class="row">
     <div class="form-inline small-12 medium-3 column end">
-      <%= f.submit t("shared.search"), class: "button" %>
+      <%= submit_tag t("shared.search"), class: "button" %>
     </div>
   </div>
 <% end %>