FIX: Remove List-Post email header (PR #14554)

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Thanks. I’m going to ask someone who is more familiar with email than me to check this out!

Why the need to (re)set it to nil when you are going to set it to either a value or nil on the next line?

Before I added the line, the same header was added twice, with the old and new value.

Right, then I think this behavior is worth adding a comment.

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