Revert "add flags to VerifySignature and sigcache"

Summary:
This reverts D2373, which was only done in the first place so that a
pre-activation 64-byte ECDSA signature could not fill the cache to
masquerade as a Schnorr signature post-activation. Now that the
upgrade is past and enable_schnorr flag is gone, this extra technical
complication is pointless (and it is unlikely to ever be useful again).

Depends on D3332

Test Plan: make check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: jasonbcox

Maniphest Tasks: T653

Differential Revision: https://reviews.bitcoinabc.org/D3254