Revert "Merge branch 'wiki-fix' into 'master'"

This reverts commit 4cc85a58 (!4026 (merged)), reversing changes made to 90ae445b.

From https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4026#note_11719950:

1. On Ruby 2.1.x, the uri.path = ::File.join(project_wiki_base_path, uri.path) raises an URI::InvalidComponentError: bad component(expected relative path component): http://bar.com/baz error in Banzai::Filter::WikiLinkFilter#rebuild_wiki_uri, which is then rescued in Banzai::Filter::WikiLinkFilter#process_link_attr, thus the link is not modified and the tests pass.
2. On Ruby 2.2.x, the same line doesn't raise an error anymore, thus tests are broken on Ruby 2.2 only. I didn't really understand yet why having absolute links in the wiki would break some tests vs having relative links as before, but I think the safest course to action is to revert this MR and to find another fix (probably just prepending ProjectWiki#wiki_base_path with the relative URL path component if we are in the case of a relative URL...

/cc @artem-sidorenko

Just a FYI note: no need to pick into 8-7-stable since the original commit was not picked.