ruby.c: remove mangled_path

* ruby.c (rubylib_mangled_path): remove obsolete code, which has
  been disabled since 5 years ago.

* man/ruby.1 (ENVIRONMENT): delete an obsolete variable to mangle
  path, RUBYLIB_PREFIX.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45599 b2dd03c8-39d4-4d8f-98ff-823fe69b080e