src: remove unused field in node_http2.h

Signed-off-by: gengjiawen <technicalcute@gmail.com>

PR-URL: https://github.com/nodejs/node/pull/25727
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

src/node_http2.h

@@ -343,9 +343,6 @@ enum session_state_flags {
   SESSION_STATE_SENDING = 0x10,
 };
 
-// This allows for 4 default-sized frames with their frame headers
-static const size_t kAllocBufferSize = 4 * (16384 + 9);
-
 typedef uint32_t(*get_setting)(nghttp2_session* session,
                                nghttp2_settings_id id);