scd:openpgp: Print a diagnostic for the use of default ECDH params.

* scd/app-openpgp.c (ecc_writekey): Remove the useless check and print
a diagnostic if the default params are used.

Note that here in 2.2 we use different default ECDH parameters than in
2.4 (AES192 instead of AES256 for 384 bit curves).

• GnuPG-bug-id: T6378