Cleanup unneeded #define in net.cpp

Summary: This code gives a bad smell. If we're going to adopt something like this, we should rename the macro to something sane and start using it everywhere. Otherwise, it just looks like lazy programming.

Test Plan: make check

Reviewers: deadalnix, schancel, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1226