Enable PKCS#3 DH in the providers

The default provider now has support for PKCS#3 Diffie-Hellman so we
switch libcrypto to using providers for that algorithm.

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/9266)

crypto/evp/exchange.c

@@ -147,10 +147,8 @@ int EVP_PKEY_derive_init_ex(EVP_PKEY_CTX *ctx, EVP_KEYEXCH *exchange)
          */
         if (ctx->pkey != NULL) {
             switch (ctx->pkey->type) {
-#if 0
             case EVP_PKEY_DH:
                 break;
-#endif
             default:
                 goto legacy;
             }