Merge #13527: policy: Remove promiscuousmempoolflags

Summary:
PR13527 backport https://github.com/bitcoin/bitcoin/pull/13527/files
faa24441ec047ec336b86f586016b9d318c1c0ad policy: Remove promiscuousmempoolflags (MarcoFalke)

Pull request description:

It seems odd to clutter validation code with features that can only ever be used for testing (testnet or regtest). Removing that test-only code makes the mempool logic less painful to understand and easier to reason about when changed or refactored in the future.

ABC note:

• promiscuousmempoolflags was originally added for segwit tests (#8149).
• adjusted dersig failure message -> mandatory
• due to large changes in D2120, the feature_cltv.py changes have been massaged into the appropriate place

Depends on D3079 and D3095

Test Plan:
make check
test_runner.py

Reviewers: jasonbcox, Fabien, #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3080