Remove generated files under wiki/ as part of cleanup
Make the wiki/ directory part of the repo

This directory is required for the site to work correctly. Rather
than rely on it being manually setup, ensure that it is always
included in a checkout and just ignore any generated *.html files
inside it.
No need to check for wiki/ directory

As the wiki/ directory is now in the repo by default, there's
no need to check for it and/or create it in the gfm.php script.