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_test.go

@@ -43,5 +43,4 @@ func TestConverterRegistry(t *testing.T) {
 	checkName("rst")
 	checkName("pandoc")
 	checkName("org")
-	checkName("blackfriday")
 }