x86/efistub: Don't try to print after ExitBootService()

setup_e820() is executed after UEFI's ExitBootService has been called.
This causes the firmware to throw an exception because the Console IO
protocol is supposed to work only during boot service environment. As
per UEFI 2.9, section 12.1:

 "This protocol is used to handle input and output of text-based
 information intended for the system user during the operation of code
 in the boot services environment."

So drop the diagnostic warning from this function. We might add back a
warning that is issued later when initializing the kernel itself.

Signed-off-by: Nikolay Borisov <nik.borisov@suse.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>