KVM: selftests: aarch64: Remove unused functions from vpmu test

KVM: selftests: aarch64: Remove unused functions from vpmu test

vpmu_counter_access's disable_counter() carries a bug that disables
all the counters that are enabled, instead of just the requested one.
Fortunately, it's not an issue as there are no callers of it. Hence,
instead of fixing it, remove the definition entirely.

Remove enable_counter() as it's unused as well.

Signed-off-by: Raghavendra Rao Ananta <rananta@google.com>
Reviewed-by: Zenghui Yu <yuzenghui@huawei.com>
Link: https://lore.kernel.org/r/20231122221526.2750966-1-rananta@google.com
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>