arm64/efi: remove idmap manipulations from UEFI code

Now that we have moved the call to SetVirtualAddressMap() to the stub,
UEFI has no use for the ID map, so we can drop the code that installs
ID mappings for UEFI memory regions.

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