std.c: Remove serenity's internet_checksum() function

See: https://github.com/SerenityOS/serenity/commit/59911d8da3da36aefd2d4902a0fdfc21e1f6f8ac
6a1d61d504Linus Groh committed on 8/10/2025, 4:21:47 PM· committed by Alex Rønne Petersenparent0a1a738
2 files changedLine totals unavailable