arch: arm64: cache: delete arm64_dcache_all

Delete arm64_dcache_all function, because it is unused and in order to
avoid future usage of set/way cache maintenance instructions. It isn't
safe to use them.

Set/way operations are not guaranteed to affect all caches prior to the
PoC, and may require other IMPLEMENTATION DEFINED maintenance.

Exposing set/way cache maintenance to virtual machines is unsafe, not
least because the instructions are not permission-checked, but also
because they are not broadcast between CPUs.

Signed-off-by: Mykola Kvach <mykola_kvach@epam.com>