Revert "update fixtures file with new concat dependencies"

This reverts commit b04d7acca5d47051e82911fee3d852a001800140.

The file_concat dependency was removed shortly after, and isn't present
in the 2.0.0 release.

.fixtures.yml

@@ -6,6 +6,5 @@ fixtures:
     stdlib: "https://github.com/puppetlabs/puppetlabs-stdlib.git"
     firewall: "https://github.com/puppetlabs/puppetlabs-firewall.git"
     concat: "https://github.com/puppetlabs/puppetlabs-concat.git"
-    file_concat: "https://github.com/electrical/puppet-lib-file_concat.git"
   symlinks:
     postgresql: "#{source_dir}"