Revert to the old TLS API, but keep certs as mg_str for DER

src/tls_openssl.h

@@ -5,15 +5,6 @@
 #include <openssl/err.h>
 #include <openssl/ssl.h>
 
-struct mg_tls_ctx {
-  X509 *server_cert;
-  EVP_PKEY *server_key;
-  STACK_OF(X509_INFO) *server_ca;
-  X509 *client_cert;
-  EVP_PKEY *client_key;
-  STACK_OF(X509_INFO) *client_ca;
-};
-
 struct mg_tls {
   SSL_CTX *ctx;
   SSL *ssl;