refactor: Remove unused BlockAssembler::pblock member var

Summary:
The difference in BlockAssembler::AddToBlock is explained by D2047:
the Bitcoin ABC codebase stores the CTransactionRef in a CBlockTemplateEntry struct, and copies it into the block later in BlockAssembler::CreateNewBlock

This is a backport of core#19283

Test Plan: ninja check-functional

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D9602