MXS-2486: Fix listener serialization

The serialization should be done via the parameters. This also makes
SSLContext::serialize redundant.

include/maxscale/ssl.hh

@@ -103,13 +103,6 @@ public:
      */
     static std::unique_ptr<SSLContext> create(const MXS_CONFIG_PARAMETER& params);
 
-    /**
-     * Serialize the SSL configuration into a INI file section
-     *
-     * @return SSLContext as a INI file section
-     */
-    std::string serialize() const;
-
     /**
      * Opens a new OpenSSL session for this configuration context
      */