Don't export Blackfriday extension front matter when Goldmark is enabled

[kaushalmodi]

Ref: #526

Goldmark markup overrides cannot work from the front matter: https://discourse.gohugo.io/t/front-matter-override-for-goldmark/26125

Also remove all Blackfriday extension tests from the test suite (ref #526 (comment)). The test site is generated with Goldmark as the default parser. So it doesn't make sense to keep the Blackfriday-specific tests.