scd:openpgp: Fallback to default ECDH params in writekey.

* scd/app-openpgp.c (ecc_writekey): Use default ECDH parameters and
remove the now useless check.

This seems to be better than bailing out. In almost all cases our
standard parameters are used and if not, well, the fingerprint will be
wrong.

• GnuPG-bug-id: T6378