trees mkpayload_grub: remove unnecessary check

these variables are initialised empty, then populated
by reading a configuration file.

it may be that in some cases, we want these variables
to be empty. besides that, the "setcfg" command before
it will throw an error if the module file is missing,
and it is assumed that the variables would be set there.

Signed-off-by: Leah Rowe <leah@libreboot.org>

script/trees

@@ -281,7 +281,6 @@ mkpayload_grub()
 {
 	eval `setvars "" grub_modules grub_install_modules`
 	eval `setcfg "$grubdata/module/$tree"`
-	chkvars "grub_install_modules" "grub_modules"
 
 	x_ rm -f "$cdir/grub.elf"