[test framework] Simplify version message handling

Summary: We do not expect to run outdated version of the software through this test framework, so hanling outdated version messages is just dead code.

Test Plan:

ninja check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D8860