src: remove unused stdlib.h include

Commit 870229e6 ("src: Add ABORT
macro") replaced the abort call with the abort macro in util-inl.h.
This commit removes the include as it is not needed anymore.

PR-URL: https://github.com/nodejs/node/pull/19427


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>