Remove show_caption_for? method

The `show_caption_for?` method was used to determine whether to check for the
presence of a `figcaption` element, but its only purpose was to skip the check
when the factory was `:budget.

The reason we skip the `figcaption` check for `:budget` is that it is the only case
where the test is verifying the form's edit page, where displaying a `figcaption` does
not make sense.

spec/system/nested_imageable_spec.rb

@@ -163,10 +163,12 @@ describe "Nested imageable" do
 
       expect(page).to have_content notice_text
 
-      visit edit_admin_budget_path(imageable) if factory == :budget
-
+      if factory == :budget
+        click_link "Go back to edit budget"
+      else
+        expect(page).to have_css "figure figcaption"
+      end
       expect(page).to have_css "figure img"
-      expect(page).to have_css "figure figcaption" if show_caption_for?(factory)
     end
 
     scenario "Different URLs for different images" do
@@ -204,10 +206,6 @@ describe "Nested imageable" do
     end
   end
 
-  def show_caption_for?(factory)
-    factory != :budget
-  end
-
   def fill_in_required_fields
     return if edit_path?