[validation] Remove REJECT code from CValidationState

Summary:

We no longer send BIP 61 REJECT messages, so there's no need to set
a REJECT code in the CValidationState object.

This is a backport of Core PR17004 [3/5]
Commit https://github.com/bitcoin/bitcoin/pull/17004/commits/e9d5a59e34ff2d538d8f5315efd9908bf24d0fdc
Depends on D8182

Test Plan: ninja all check-all

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8183