topology: Remove unused cpu_cluster_mask()

default_topology[] uses cpu_clustergroup_mask() for the CLS level
(guarded by CONFIG_SCHED_CLUSTER) which is currently provided by x86
(arch/x86/kernel/smpboot.c) and arm64 (drivers/base/arch_topology.c).

Fixes: 778c558f

 ("sched: Add cluster scheduler level in core and
related Kconfig for ARM64")

Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Barry Song <baohua@kernel.org>
Link: https://lore.kernel.org/r/20220513093433.425163-1-dietmar.eggemann@arm.com