Fix DH/DHX named groups to not overwrite the private key length.

Fix DH/DHX named groups to not overwrite the private key length.

The only reason(s) the DH private key length should be set are:
(1) The user sets it during key generation via EVP_PKEY_CTX_set_params
    using OSSL_PKEY_PARAM_DH_PRIV_LEN.
(2) When loading a PKCS3 (DH) key the optional value
    'privateValueLength' is set.

Now that the named groups contain a value for 'q' there is no reason to
automatically overwrite the private key length.

Issue detected by @davidmakepeace

Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15695)