Don't create bundle header for header-less page resource content (e.g. md)

[bep]

Given these files:

-- hugo.toml --
baseURL = "https://example.com"
disableKinds = ["taxonomy", "term"]
defaultContentLanguage = "en"
defaultContentLanguageInSubDir = true
[languages]
[languages.nn]
weight = 2
[languages.en]
weight = 1
-- content/section/mybundle/index.md --
---
title: "Mybundle"
---
-- content/section/mybundle/bundledpage.md --
---
title: "Bundled page en"
---
-- content/section/mybundle/bundledpage.nn.md --
---
title: "Bundled page nn"
---

-- layouts/_default/single.html --
Bundled page: {{ .RelPermalink}}|Len resources: {{ len .Resources }}|

In the current Hugo we have some logic that clones the content/section/mybundle/index.md for the nn language to make the bundled page content/section/mybundle/bundledpage.nn.md work.

This logic isn't something we can/want to maintain, especially when we consider getting content from other data sources.

Ref. #6136