fix AreInputsStandard sigops counting
Summary:
There is a better sigops counter for P2SH already provided (the
one used by consensus code), and also along the lines of D5061
and D5076 we want all sigops counts to be contextual, never using
internal hardcoded flags.
Test Plan:
ninja bench-bitcoin check-all
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Maniphest Tasks: T704
Differential Revision: https://reviews.bitcoinabc.org/D5160