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.

PR-URL: https://github.com/nodejs/node/pull/24713


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>