src: remove BeforeExit callback list

It obscures the fact that there is only a single BeforeExit action.
Just call that statically from `EmitBeforeExit()`.

PR-URL: https://github.com/nodejs/node/pull/33386


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>