GRUB: drop `insmod vbe`

When booting in EFI mode this error message is visible for a
very short period of time:

| error: file `/boot/grub/x86_64-efi/vbe.mod' not found

This is annoying, especially because it's hard to read and identify.

It's for sure not relevant within Secure Boot mode, as it's forbidden
there. Let's also disable it by default for Non-Secure-Boot-mode,
if it's relevant for anyone or on specific systems we'll find out.