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