Do not check for non replay protected transaction in CheckInputs

Summary: Now that we do replay protected txns only, this can go.

Test Plan:

make check
../qa/pull-tester/rpc-tests.py

Reviewers: freetrader, CCulianu, hanchon, #bitcoin_abc, sickpig, thezerg, awemany

Reviewed By: CCulianu, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D442