Revert "Support sys-boot/grub-2.02_beta2"

This reverts commit a61c6fa08ce88cc2b9659eb8714cafc2292d692d.
Handle the /boot/grub2/grub.cfg -> /boot/grub/grub.cfg move in Gentoo

Signed-off-by: Richard Yao <ryao@gentoo.org>