Add CHECKDATASIG to standard flags.

Summary: Also remove STANDARD_CHECKDATASIG_VERIFY_FLAGS.

Test Plan:
make check
Also did make check with test change only, and with code change only,
to check that it fails as expected.
Did IBD on mainnet and testnet with -checkpoints=0 -assumevalid=0

Depends on D3063

Reviewers: deadalnix, Fabien, #bitcoin_abc, markblundeberg, jasonbcox

Reviewed By: deadalnix, #bitcoin_abc, markblundeberg, jasonbcox

Subscribers: markblundeberg

Maniphest Tasks: T653

Differential Revision: https://reviews.bitcoinabc.org/D3069