This reverts commit abf8de85.
We should take a deeper look to those cases flat_map doesn't do deep
flattening.

irb(main):002:0> [[[1,3], [1,2]]].map{|i| i}.flatten
=> [1, 3, 1, 2]
irb(main):003:0> [[[1,3], [1,2]]].flat_map{|i| i}
=> [[1, 3], [1, 2]]