Use the normal state changed `HookableEvent` instead of having
`Core::NotifyStateChanged` call `g_perf_metrics.OnEmulationStateChanged`
directly.
The direct call was added in bad78cfed4 to
avoid a crash. At the time state changed callbacks were stored in a
vector, and the crash was caused by `g_perf_metric`'s destructor trying
to remove the callback from the already-destroyed vector.
Later a97627e736 switched state changed
callbacks to use `HookableEvent`, which is specifically designed to
handle the case where a hook outlives its associated event.
Since the workaround is no longer necessary replace it with a standard
`EventHook`.
252ec7452c