arm64/efi: drop redundant set_bit(EFI_CONFIG_TABLES)

The EFI_CONFIG_TABLES bit already gets set by efi_config_init(),
so there is no reason to set it again after this function returns
successfully.

Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>