src: remove unused context variable in node_serdes

Currently the following compiler warnings is generated:

../src/node_serdes.cc:400:18:
warning: unused variable 'context' [-Wunused-variable]
  Local<Context> context = ctx->env()->context();
                 ^
1 warning generated.

This commit removes the unused variable.

Checklist

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