PerformanceMetrics: Use HookableEvent for state changed callback

Use the normal state changed `HookableEvent` instead of having
`Core::NotifyStateChanged` call `g_perf_metrics.OnEmulationStateChanged`
directly.

The direct call was added in bad78cfed416d89ceb6b0acf6c134d3691b3d624 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 a97627e736ff352e031123519c799e65cc98b32c 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`.

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