Remove m_is_manual_connection from CNodeState

Summary:
Currently, this member is only used to exclude MANUAL peers from discouragement
in MaybePunishNodeForBlock(). Manual connections are already protected in
MaybeDiscourageAndDisconnect(), independently from their network
processing behaviors.

This is a backport of core#20617

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10946