Remove leftover short-circuit evaluation condition from bdb377d68ea8

Summary:
Removes extra stuff left over from cleanup in bdb377d68ea8,
"Kill SegWit Step 11: Remove HasWitness".

Test Plan: This fixes the code to evalulate like "tx.HasWitness() == false" holds. Compare also bdb377d68ea8.

Reviewers: deadalnix, freetrader, sickpig, kyuupichan, O1 Bticoin ABC, #bitcoin_abc

Reviewed By: deadalnix, O1 Bticoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D185