(maint) Remove stale files from ext/rack

NOTE FOR REVIEWER: Does any packaging script or anything automatically grab the ext/rack/files/config.ru file? If so, moving it might be a bad idea. I didn't know off the top of my head how to find out.

The ext/rack directory had an outdated README file and an ancient manifest.pp
file, and the useful files (config.ru and the example vhost) were buried in a
subdirectory.

This commit removes the old files, moves the good files up a level, and adds a
comment to the vhost pointing to the maintained passenger documentation.