arm64: mm: remove unused __cpu_uses_extended_idmap[_level()]

These routines lost all existing users during the latest merge window so
we can remove them. This avoids the need to fix them in the context of
fixing a regression related to the ID map on 52-bit VA kernels.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20210310171515.416643-3-ardb@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>