Cleanup of dead mapBlockSource entries that aren't needed in FinalizeNode

Summary: A tiny memory leak exists in net_processing.cpp, this patch fixes it

Test Plan: Compile and run bitcoin. No log messages are generated to verify this is working, but if you wish, you can add some to FinalizeNode() to see stale mapBlockSource entries accumulate as bitcoin runs.

Reviewers: deadalnix, freetrader, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D372