Remove grub-legacy support

Following @fzielcke's announce about the removal of grub-legacy from Debian, this MR provides a patch removing grub-legacy support from grub-installer.

As mentioned in https://lists.debian.org/debian-boot/2024/03/msg00013.html grub-installer still forces grub-legacy if the boot device is on multipath, although grub2 is supposed to support it now.

However I am not able to test with multipath either.

This MR also provides patches for additional cleanup.

Closes: #1086750 #1086840