Remove redundant "loading-bar.errors"

In the imageable_attach_new_file method used in this tests the:
> expect(page).to have_css ".loading-bar.errors"
is already being checked.

Therefore, to leave only the line:
> imageable_attach_new_file(file_fixture("logo_header.png"), false)
in the test, since there is another test that verifies it,
I think we can remove the test altogether.

spec/system/nested_imageable_spec.rb

@@ -81,12 +81,6 @@ describe "Nested imageable" do
       end
     end
 
-    scenario "Should update loading bar style after invalid file upload" do
-      imageable_attach_new_file(file_fixture("logo_header.png"), false)
-
-      expect(page).to have_css ".loading-bar.errors"
-    end
-
     scenario "Should update image cached_attachment field after valid file upload" do
       click_link "Add image"