remove cs_main extern declaration from net_processing.h

Summary:
net_processing.h does not need cs_main since D10890
net_processing.cpp includes validation.h where cs_main is defined.

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13322