Remove ::GetVirtualTransactionSize() alias

Summary:
Each alias is only used in one place.

This is a partial backport of core#25648
https://github.com/bitcoin/bitcoin/pull/25648/commits/fa477d32eefcc3dd2f06b452066290d9936d8c5d

Depends on D15757

Test Plan: ninja all check-all bitcoin-fuzzers

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15777