drm/i915/debugfs: Remove i915_energy_uJ

Last IGT user (intel_gpu_overlay) stopped using it in October 2019 so we
are good to remove the file.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20200206140725.10973-1-tvrtko.ursulin@linux.intel.com