Undefine SimpleDelegator#open

Including `::Kernel.dup` brings across the #open method into
SimpleDelegator with undesirable consequences.

e.g.

```Ruby
  class A
  end
  SimpleDelegator.new(A.new).open
  # => NoMethodError: private method `open' called for #<A:0x007fca1aa1fbd0>
```