Extract budgets from shared nested imageable specs to system specs

Removed `imageable_path_arguments`, `has_many_images`, and `management` parameters
because they are not used by budgets.

Hardcoded `path`, `fill_resource_method_name`, `submit_button`, and
`imageable_success_notice`parameters for budgets. These remain fixed for now until dynamic
values are required in the next commits.

spec/system/admin/budgets_spec.rb

@@ -1,14 +1,6 @@
 require "rails_helper"
 
 describe "Admin budgets", :admin do
-  it_behaves_like "nested imageable",
-                  "budget",
-                  "new_admin_budgets_wizard_budget_path",
-                  {},
-                  "imageable_fill_new_valid_budget",
-                  "Continue to groups",
-                  "New participatory budget created successfully!"
-
   context "Load" do
     before { create(:budget, slug: "budget_slug") }