Fix handling of HTML files without front matter

This means that any HTML file inside /content will be treated as a regular file.

If you want it processes with shortcodes and a layout, add front matter.

The defintion of an HTML file here is:

* File with extension .htm or .html
* With first non-whitespace character "<" that isn't a HTML comment.

This is in line with the documentation.

Fixes #7030
Fixes #7028
See #6789

hugolib/page__content.go

@@ -28,7 +28,6 @@ var (
 
 // The content related items on a Page.
 type pageContent struct {
-	renderable bool
 	selfLayout string
 	truncated  bool
 
@@ -52,7 +51,7 @@ func (p pageContent) contentToRender(renderedShortcodes map[string]string) []byt
 		case pageContentReplacement:
 			c = append(c, v.val...)
 		case *shortcode:
-			if !p.renderable || !v.insertPlaceholder() {
+			if !v.insertPlaceholder() {
 				// Insert the rendered shortcode.
 				renderedShortcode, found := renderedShortcodes[v.placeholder]
 				if !found {