drm/i915/pmu: Remove pointless synchronize_rcu() call

This is already done inside perf_pmu_unregister() - no need to do it
before.

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241104213512.2314930-5-lucas.demarchi@intel.com


Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>