Revert "Preload lib/"

This reverts commit 5511a731.

The original commit added this because it also enabled threadsafety, a change which was itself later reverted in 66d6c809, but this got left behind.

I don't fully understand the reasoning behind it so if I'm wrong, please tell me.

My reasoning for reverting it is that it messes with Rails' (and by extension Spring's) class reloading during development. When I was working in `lib/gitlab/markdown` and had to stop and restart the server every time I made a change, I didn't know it at the time, but this was why. That was a huge pain point.

If it's needed for production perhaps we could add a `if Rails.env.production?` clause around it so that it doesn't mess with development.

See merge request !1758