crypto: ecc - delete a useless function declaration

This function declaration has been added in 'ecc_curve.h',
delete it in 'crypto/ecc.h'.

Fixes: 4e660291(crypto: ecdsa - Add support for ECDSA ...)
Signed-off-by: Meng Yu <yumeng18@huawei.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>