Follow-up !2369: merge an iter and a fold in P2p_acl.IPGreylist.remove_old

The following discussion from !2369 (merged) should be addressed:

@igarnier started a discussion: (+1 comment)

Couldn't we merge this fold with the List.iter below, ie remove elements from the FIFO in the fold?

We need to release the Ringo library with the filter function.

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