UPSTREAM: arm64: mm: kill mm_cpumask usage

mm_cpumask isn't actually used for anything on arm64, so remove all the
code trying to keep it up-to-date.

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

Bug: 31432001
Change-Id: Ib33459092474fdc45dd8bb7e21658804c778fdb8
(cherry picked from commit 38d96287)
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>