When adding SignatureBits control logic, we incorrectly allowed
specification of SignatureBits in the case of an ECDSA issuer. As noted
in the original request, NIST and Mozilla (and others) are fairly
prescriptive in the choice of signatures (matching the size of the
NIST P-curve), and we shouldn't usually use a smaller (or worse, larger
and truncate!) hash.

Ignore the configuration of signature bits and always use autodetection
for ECDSA like ed25519.
Signed-off-by: Alexander Scheel <alex.scheel@hashicorp.com>