MAINTAINERS: adjust entry after arm64 efi-entry.S file movement

Commit 6ed27461c9c8 ("arm64: efi: Move efi-entry.S into the libstub source
directory") moves arch/arm64/kernel/efi-entry.S to
drivers/firmware/efi/libstub/arm64-entry.S, but misses to adjust the
MAINTAINERS file.

Hence, ./scripts/get_maintainer.pl --self-test=patterns complains about a
broken file pattern.

Adjust the file entry in EXTENSIBLE FIRMWARE INTERFACE (EFI) to this file
movement.

As the file drivers/firmware/efi/libstub/arm64-entry.S is already covered
by the entry drivers/firmware/efi/, simply remove the arm64 file entry.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>