Remove ldlinux.c32 for extlinux and syslinux
Since syslinux 5.00, this file is installed into the disk root by the extlinux and syslinux installer commands. For pxelinux and isolinux, these files still need to be added manually. This is documented at:
Keeping these unused files around is probably only confusing, so better to remove them.
This commit was previously part of MR !19, but is not really required for that, so I moved it into this separate MR instead.
I've tested booting in QEMU with an isolinux isohybrid image (sudo lb config -d testing), syslinux hdd image (sudo lb config -b hdd -d testing --bootloaders syslinux) and extlinux hdd image (sudo lb config -b hdd -d testing --bootloaders syslinux --binary-filesystem ext2). I used Debian testing since IIRC there was a bug in the stable syslinux version (could be syslinux-efi-only, though). I specified the bootloaders explicitly for hdd images, since otherwise it includes grub-efi by default which is not supported for hdd images (this is fixed by !19).