src: fix compiler warnings in node_http2.cc

Currently, the following compiler warnings are generated:
../src/node_http2.cc: In static member function
‘static int node::http2::Http2Session::OnStreamClose(nghttp2_session*,
    int32_t, uint32_t, void*)’:
../src/node_http2.cc:994:16: warning:
variable ‘def’ set but not used [-Wunused-but-set-variable]
  994 |   Local<Value> def = v8::False(env->isolate());
      |                ^~~
../src/node_http2.cc: In static member function
‘static void node::http2::Http2Session::Ping(
    const v8::FunctionCallbackInfo<v8::Value>&)’:
../src/node_http2.cc:2755:16: warning:
unused variable ‘env’ [-Wunused-variable]
 2755 |   Environment* env = Environment::GetCurrent(args);
      |                ^~~
../src/node_http2.cc: In static member function
‘static void node::http2::Http2Session::Settings(
const v8::FunctionCallbackInfo<v8::Value>&)’:
../src/node_http2.cc:2774:16: warning:
unused variable ‘env’ [-Wunused-variable]
 2774 |   Environment* env = Environment::GetCurrent(args);
      |                ^~~

This commit removes these unused variables.

PR-URL: https://github.com/nodejs/node/pull/33014


Reviewed-By: Zeyu Yang <himself65@outlook.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Juan José Arboleda <soyjuanarbol@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>