src: remove unnecessary forward declaration

I can't see that the forward declaration of class Connection is needed
and wanted to raise this in case it was overlooked after a previous
change.

PR-URL: https://github.com/nodejs/node/pull/13081
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

src/node_crypto.h

@@ -88,9 +88,6 @@ extern X509_STORE* root_cert_store;
 
 extern void UseExtraCaCerts(const std::string& file);
 
-// Forward declaration
-class Connection;
-
 class SecureContext : public BaseObject {
  public:
   ~SecureContext() override {