src: remove unused freelist.h header

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

PR-URL: #20544
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Ben Noordhuis <[email protected]>
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Daniel Bevenius <[email protected]>
Reviewed-By: Matheus Marchini <[email protected]>
Reviewed-By: Tiancheng "Timothy" Gu <[email protected]>