src: remove unused freelist.h header

Always easy enough to re-introduce if we do need it.

PR-URL: #20544
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Matheus Marchini <matheus@sthima.com>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>

Author: addaleax