Remove dead code in duplicable.rb

[casperisfine]

Edit: #28544

AFAIK all these feature tests are for pre 2.5.0 MRI, now that Rails requires 2.5.0+, it's no longer useful to test it.

While I was at it I added one exception for UnboundMethod that was missing.

Another class that could use it is Thread:

>> Thread.new{}.dup
TypeError (allocator undefined for Thread)

However it would mean requiring the thread module, which seems wrong.

cc @rafaelfranca @Edouard-chin

[Edouard-chin]

👍

[kamipo]

Can you add a test for UnboundMethod here?

rails/activesupport/test/core_ext/object/duplicable_test.rb

Line 9 in d238091

RAISE_DUP = [method(:puts)]

[casperisfine]

@kamipo done!