efi/libstub: Simplify "Exiting bootservices" message

The message

    "Exiting boot services and installing virtual address map...\n"

is even shown if we have efi=novamap on the command line or the firmware
does not provide EFI_RT_SUPPORTED_SET_VIRTUAL_ADDRESS_MAP.

To avoid confusion just print

    "Exiting boot services...\n"

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>