Rename GetLegacySigOpCount into GetSigOpCountWithoutP2SH
Summary: So it's actually clear what it does.
Test Plan:
make check
Reviewers: freetrader, sickpig, #bitcoin_abc, kyuupichan
Reviewed By: #bitcoin_abc, kyuupichan
Subscribers: kyuupichan
Differential Revision: https://reviews.bitcoinabc.org/D128