src: remove unused variable in node_contextify

Currently the following warning is show when building:
../src/node_contextify.cc:642:10:
warning: unused variable 'display_errors' [-Wunused-variable]

    bool display_errors = maybe_display_errors.ToChecked();

This commit removes the unused variable which became unused in commit
aeddc367 ("src: remove tracking for
exception arrow data").

PR-URL: https://github.com/nodejs/node/pull/17491
Refs: https://github.com/nodejs/node/pull/17394


Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>