src: remove unused variables

Currently the following compiler warnings are generated:

../src/node_process.cc:799:16:
warning: unused variable 'ary' [-Wunused-variable]
  Local<Array> ary = Array::New(args.GetIsolate());
               ^
1 warning generated.

../src/node_http2.cc:1294:16:
warning: unused variable 'holder' [-Wunused-variable]
  Local<Array> holder = Array::New(isolate);
               ^
1 warning generated.

This commit removes these unused variables.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines