Revert "Deprecate the `eager_load_paths` configuration"

Because of the possibility of lib being unintentionally eager loaded
it's been agreed that we'll leave autoload paths and eager load paths
separate for Rails 4.0.

This reverts commit 0757b3388ffe4f44b60de950d40e18ef05055931.

Conflicts:
	railties/CHANGELOG.md
Don't unhook autoloading in production

Whilst autoloading is known to be not threadsafe, leaving it in place
is a softer solution than failing hard when an application is deployed.

Many older applications will have paths added to `autoload_paths` and
ideally these should be eagerly loaded to be threadsafe. However one
of these paths is quite often lib which could lead to unintended
consequences due to the 'junk drawer' nature of this directory.

Developers should refrain from adding paths to `autoload_paths` or
`eager_load_paths` and use custom folders inside app for code that
needs to be eagerly loaded and use `require` or `require_dependency`
to explicitly load code from other locations.
Remove config.autoload_paths from config/application.rb

Most application developers shouldn't be adding paths to
`config.autoload_paths` as it won't be eagerly loaded so
remove it from the generated config/application.rb file.

If a developer does want to autoload from paths outside of the app
directory then they should add the path to `config.eager_load_paths`
as these paths are also autoloaded as well.