Disable ed25519-donna's batch verification.

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.
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