Add instruction to remove an unused 2fa key

[nirdosh17]

We can have volatile staging environments where 2fa is enable but is valid for couple of weeks only and not usable for that and we need to create new 2fa. In such case, the old 2fa keys just remain there and create a mess. So, i have added instruction to delete the 2fa key.

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[nirdosh17]

@googlebot I signed it!

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