arm64: perf: remove misleading comment

The comment for the armv8pmu_set_event_filter function suggests that
it only works for PMUv2 PMUs - this is incorrect.

Let's remove the incorrect comment.

Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Andrew Murray <andrew.murray@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>