remove segwit 'sigops cost' leftovers
Summary:
In the segwit PR 8149, sigops count was quadrupled to become 'sigops cost',
Several places were still using the quadrupled value and the 'sigops cost'
name too.
https://github.com/bitcoin/bitcoin/pull/8149/commits/2b1f6f9ccf36f1e0a2c9d99154e1642f796d7c2b
Test Plan: ninja all check bench-bitcoin
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D4898