http2: delete unused enum in node_http2.h

PR-URL: #26704
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>

src/node_http2.h

@@ -55,10 +55,6 @@ enum nghttp2_session_type {
   NGHTTP2_SESSION_CLIENT
 };
 
-enum nghttp2_shutdown_flags {
-  NGHTTP2_SHUTDOWN_FLAG_GRACEFUL
-};
-
 enum nghttp2_stream_flags {
   NGHTTP2_STREAM_FLAG_NONE = 0x0,
   // Writable side has ended