adapt the location for grub2 efi binaries to (modern) reality

We no longer (>= 2013-01) override the install path to
	/boot/efi/EFI/aptosid/grubx64.efi
and keep the default at
	/boot/efi/EFI/debian/grubx64.efi

Signed-off-by: Stefan Lippers-Hollmann <s.l-h@gmx.de>