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").

Refs: https://github.com/nodejs/node/pull/17394

Checklist

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

Affected core subsystem(s)

src