Fixes bug 40078.
As reported by hdevalence our batch verification logic can cause an assert crash.
The assert happens because when the batch verification of ed25519-donna fails,
the code in ed25519_checksig_batch() falls back to doing a single
verification for each signature.
The crash occurs because batch verification failed, but then all signatures individually verified just fine.
That's because batch verification and single verification use a different equation which means that there are sigs that can pass single verification but fail batch verification.
Fixing this would require modding ed25519-donna which is not in scope for this ticket, and will be soon deprecated in favor of arti and ed25519-dalek, so my branch instead removes batch verification.