Merge #14025: p2p: Remove dead code for nVersion=10300

Summary:
fa74d3d720 qa: Remove unused deserialization code in msg_version (MarcoFalke)
fa5099ceb7 p2p: Remove dead code for nVersion=10300 (MarcoFalke)

Pull request description:

This code is undocumented and confusing as well as dead, since peers with a version that old are disconnected immediately.

Tree-SHA512: 58c131a2730b630ffdc191cd65fe736ed1bd57e184902e2af1b1399443c4654617e68774432016df023434055e85d2e8cd32fb03b40c508c3bb8db6d19427434

Backport of Core PR14025
https://github.com/bitcoin/bitcoin/pull/14025/

Also removed relevant obsolete code from seeder.

Reviewer note: the net_processing changes we made in D333

Test Plan:

make check
test_runner.py
./bitcoin-seeder

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: deadalnix, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4449