src: remove unused isolate variable

Currently the following compiler warning is generated:

../src/node_task_queue.cc:93:12: warning:
unused variable 'isolate' [-Wunused-variable]
  Isolate* isolate = env->isolate();
           ^
1 warning generated.

This commit removes the unused variable.

Checklist

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