Remove unused AV/test/fixtures/blog_public

The fixtures were added to support StaticTests in 401cd97 but
those tests were then removed in d5ad92ce.

If Travis CI is happy with this PR, then you can be sure that
those fixtures are not needed anymore.
Merge pull request #17871 from claudiob/remove-unused-blog-public-fixtures

Remove unused AV/test/fixtures/blog_public