Delete duplicate keys on :pop in Keyword.get_and_update/3 and get_and_update!/3

[pnezis]

Both functions document updating "in one pass, deleting duplicate keys", but the :pop branch left any later duplicates in place:

iex> Keyword.get_and_update([a: 1, a: 2], :a, fn _ -> :pop end)
{1, [a: 2]}

I am evaluating claude fable on code review tasks and this was one of the findings

[josevalim]

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