Revert "Short circuit the scoping delegation for relation.all"

This reverts commit eb80738.

If the current scope is affected by the `unscoped` block, `all` won't be
the same with `spawn`.

Author: kamipo

activerecord/lib/active_record/relation/spawn_methods.rb

@@ -10,7 +10,6 @@ module SpawnMethods
     def spawn #:nodoc:
       clone
     end
-    alias :all :spawn
 
     # Merges in the conditions from <tt>other</tt>, if <tt>other</tt> is an ActiveRecord::Relation.
     # Returns an array representing the intersection of the resulting records with <tt>other</tt>, if <tt>other</tt> is an array.