simplify ATMP standard flag computation [1/2] - remove CHECKDATASIG_SIGOPS

Summary:
This flag is already part of STANDARD_SCRIPT_VERIFY_FLAGS, and anyway
it doesn't matter here since CheckInputs does not care about sigops.

Test Plan: ninja check

Reviewers: Mengerian, #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5014