Remove SYSLINUX from USB images: use GRUB for legacy boot

Low priority but I wonder whether we could get rid of SYSLINUX on USB images.

I think I remember that it supports isohybrid which works fine on DVD and getting GRUB to work on DVD might be tricky (and definitely not worth it), but I wonder what prevents us from always using GRUB on USB images.

I admit that the benefits are thin:

• Make our installation and boot options instructions simpler but removing an if clause.
• Provide #17641 equally to everybody (once we get there).

One day I'll put again on the table the idea to stop shipping ISO images and then we could get rid of SYSLINUX for everybody. But I'll wait a bit more 😁