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.

PR-URL: https://github.com/nodejs/node/pull/25368


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>