efi: Remove unused extern declaration efi_lookup_mapped_addr()

Since commit 50a0cb56 ("x86/efi-bgrt: Fix kernel panic when mapping BGRT data")
this extern declaration is not used anymore.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>