Remove SCRIPT_VERIFY_CHECKDATASIG_SIGOPS flag

Summary:
This flag is unused since D6101

Port of BCHN MR#748

As remarked in its definition it is now a meaningless flag
now that sigchecks upgrade is complete (and sigops no longer
exist).

(Historically this flag was used to enable the opcode, then
after the CDS upgrade it became a control on the sigops counting
only. Now after sigchecks it is meaningless.)

Test Plan:
git grep SCRIPT_VERIFY_CHECKDATASIG_SIGOPS
ninja check
Did IBD, checked that blockchain synced to same block as public explorers.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11102