Remove Blackfriday markdown engine

It has been deprecated for a long time, its v1 version is not maintained anymore, and there are many known issues. Goldmark should be
a mature replacement by now.

Closes #9934

markup/markup_config/config.go

@@ -18,7 +18,6 @@ import (
 	"github.com/gohugoio/hugo/config"
 	"github.com/gohugoio/hugo/docshelper"
 	"github.com/gohugoio/hugo/markup/asciidocext/asciidocext_config"
-	"github.com/gohugoio/hugo/markup/blackfriday/blackfriday_config"
 	"github.com/gohugoio/hugo/markup/goldmark/goldmark_config"
 	"github.com/gohugoio/hugo/markup/highlight"
 	"github.com/gohugoio/hugo/markup/tableofcontents"
@@ -37,8 +36,6 @@ type Config struct {
 
 	// Content renderers
 	Goldmark    goldmark_config.Config
-	BlackFriday blackfriday_config.Config
-
 	AsciidocExt asciidocext_config.Config
 }
 
@@ -56,10 +53,6 @@ func Decode(cfg config.Provider) (conf Config, err error) {
 		return
 	}
 
-	if err = applyLegacyConfig(cfg, &conf); err != nil {
-		return
-	}
-
 	if err = highlight.ApplyLegacyConfig(cfg, &conf.Highlight); err != nil {
 		return
 	}
@@ -83,26 +76,6 @@ func normalizeConfig(m map[string]any) {
 	}
 }
 
-func applyLegacyConfig(cfg config.Provider, conf *Config) error {
-	if bm := cfg.GetStringMap("blackfriday"); bm != nil {
-		// Legacy top level blackfriday config.
-		err := mapstructure.WeakDecode(bm, &conf.BlackFriday)
-		if err != nil {
-			return err
-		}
-	}
-
-	if conf.BlackFriday.FootnoteAnchorPrefix == "" {
-		conf.BlackFriday.FootnoteAnchorPrefix = cfg.GetString("footnoteAnchorPrefix")
-	}
-
-	if conf.BlackFriday.FootnoteReturnLinkContents == "" {
-		conf.BlackFriday.FootnoteReturnLinkContents = cfg.GetString("footnoteReturnLinkContents")
-	}
-
-	return nil
-}
-
 var Default = Config{
 	DefaultMarkdownHandler: "goldmark",
 
@@ -110,8 +83,6 @@ var Default = Config{
 	Highlight:       highlight.DefaultConfig,
 
 	Goldmark:    goldmark_config.Default,
-	BlackFriday: blackfriday_config.Default,
-
 	AsciidocExt: asciidocext_config.Default,
 }