src: remove unused variable in node_file

Currently the following warning is reported when building:

../src/node_file.cc:542:18:
warning: unused variable 'context' [-Wunused-variable]
  Local<Context> context = env->context();
                 ^
../src/node_file.cc:566:18:
warning: unused variable 'context' [-Wunused-variable]
  Local<Context> context = env->context();
                 ^
2 warnings generated.

The commit removes the unused variables.

Checklist

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

Affected core subsystem(s)

src