http_server: remove unused variable

|useSsl| was not referenced anywhere else, so delete it.

Tested: code compiles.

Signed-off-by: Nan Zhou <nanzhoumails@gmail.com>
Change-Id: Ie6ba9c122ab0897254ed3cb0e278d43e8b6a283c
diff --git a/http/http_server.hpp b/http/http_server.hpp
index d1a8ed3..a0ccc09 100644
--- a/http/http_server.hpp
+++ b/http/http_server.hpp
@@ -202,9 +202,6 @@
 
     Handler* handler;
 
-#ifdef BMCWEB_ENABLE_SSL
-    bool useSsl{false};
-#endif
     std::shared_ptr<boost::asio::ssl::context> adaptorCtx;
 };
 } // namespace crow