refactor: remove unused header <arpa/inet.h> in protocol.cpp

Summary:

There is no code using types or functions related to "internet operations" anymore in protocol.cpp (since #735, more than 8 years ago!), hence the header include can be removed.

This is a backport of core#19890

Test Plan: ninja

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10184