agent: Fix importing ECC key.
* agent/cvt-openpgp.c (convert_from_openpgp_main): Only encrypted parameters are stored as opaque. (apply_protection): ARRAY members are all normal, non-opaque MPI. (extract_private_key): Get public key as normal, non-opaque MPI. Remove support of ECC key with '(flags param)'. Remove support of "ecdsa" and "ecdh" keys of our experiment.